Dvarapalas are regular features of a major Hindu or Buddhist temple complex. They are the formidable looking ‘gate-keepers’ and guards in service of the presiding deity of the temple. They are the servants and the protectors of their masters. They are typically envisioned as huge and robust warriors. The pairs of Dvarapalas are most usually placed at the entrance to the temple and also at the door way to sanctum (garbha-griha). As you mentioned, each god or goddess has his or her own set of Dvarapalas.
2.1. Dvarapalas are classified as parivara-devathas, meaning that Dvarapalas are semi-divine beings of a minor class who form the entourage of the main deity they serve. The Shilpa Sastra texts that deal with temple architecture (devalaya-vastu) after describing the temple layout, structure and other aspects with particular reference to the attributes and disposition of the deity to be installed in the temple , do make a mention of the nature and appearances of the Dvarapalas to be placed at different locations in the temple complex. There are in addition, numerous Dhyana-slokas, or word-pictures in verse that present graphic details of the form, substance and attributes of the deietyand his or her attendants. These verses are meant for contemplation and guidance of the Shilpi, the sculptor.I do not know if there are any texts that deal exclusively with the depiction of the Dvarapalas. They form a detail of the larger picture.
2.2. Since Dvarapalas are parivara-devathas, their appearance, attributes etc have to be in accordance with that of their Master, the principal deity that resides in the sanctum. Therefore their costume, weapons, insignia or emblems are indicative of the powers, virtues and magnificence of the presiding deity. Their appearances and stance herald the nature and disposition of the main deity; and also the affiliation of the temple- such as Shiva, Vishnu, and Devi etc.
3.1. Accordingly, the Dvarapalas in a Vishnu temple are rather placid looking; modestly dressed and ornamented. They are adorned with the signs and emblems of Vishnu such as the tilaka on their fore heads and urdhvapundra (Nama) on their faces, arms, chest etc.. They carry in their upper hands the conch (shankha) and discus (chakra); and in the lower hands, the mace (gada) and a noose (pasha, coil of rope). They stand erect, cross-legged leaning on their mace as if they are resting. The gestures of their fingers and the look in their eyes caution one to behave properly in the presence of the divinity. The nature and appearance of the Dvarapalas of Vishnu are described in the Agama texts: Isvara Samhita and Pushkara Samhita.
3.2. Similarly, the Dvarapalas in a Shiva temple take after Virabhadra, the ferocious aspect of Shiva. They look fierce with bulging eyes, protruding curved sharp canine teeth, horns (at times); and with their threatening stance and fearsome weapons. They have thick mustaches, bushy eyebrows and hairy abdomen. They wear the emblems of Shiva, such as the stripes of ash, animal hides, long flowing unkempt hair etc. They carry a trident, mace, broad-sword and a noose. They look ferocious, gesture ominously and stand planting firmly a foot on the mace. The features of the Dvarapalas of Shiva are described in the latter part (uttarardha) of Kashyapa Shilpa Sastra.
3.3. In the Shaktha tradition where the distinctions between the gross and subtle forms are marked and sharp, the Dvarapalas of the female deities who represent the grosser elements of nature are fearsome looking females, modeled after the ferocious aspect of their Mother deity. They carry cutlasses and tridents; wear garlands of skull; and sport wild unkempt hair. Quite often they are portrayed with flashy eyes, long protruding teeth and tongue spread out of the open mouth. The Dvarapalas of the Devi are pictured in Kalika Puranam.
n the Dakshina-chara School (the right handed method) of Sri Vidya tradition the guarding deities are the physical (sthula) representations of certain symbolic concepts. For instance, the outermost enclosure (avarana) of Sri Chakra, named Bhupura Chakra - the earth stretch, has four gates (dvara). The Eastern gate is the way of the mantras; the Southern gate is the way of devotion or bhakti; the Western gate is for the performance of rites and rituals, or karma-kanda.; and the Northern gate is the way of wisdom, or Jnana. The Mudra devathas, the standard bearers, the approach to the divinities and carrying seals of authority, guard those entrances. They resemble in appearance the auspicious form of the Mother Goddess and carry weapons such as bow, arrows, goad and noose.
4.1. As regards the general features of all Dvarapalas placed in the temples, they are well built, muscular, broad shouldered and very tall. They are always soldier-like and larger than life; but they can hardly be called very terrifying. The Dvarapala are not provided with halos or garlands. They always carry weapons; and are always depicted as standing guard. Dvarapalas are always in pair or in even numbers.The Agama texts recommend four pairs of Dvarapalas, each pair to guard a cardinal direction. The Dvarapala images are usually scaled in saptha (seven) tala or nava (nine) tala measure. They are made either with two or four arms.
4.2. The Dvarapalas, in each case, are in some way associated with their main deity through a legend detailed in a Purana. The Dvarapalas of major deities such as Vishnu or Shiva have recognizable names and specified positions. In the Agamas they are termed Ganeshvara, the chief of the horde.
For instance the four pairs of Dvarapalas of Vishnu are (i) Chanda and Prachanda ;( ii) Dhatru and Vidhatru; (iii) Jaya and Vijaya; and (iv) Bhardra and Subhadra. The first named in each pair stands to the right of the doorway; and the other to the left.
Similarly, the Dvarapalas of Shiva are (i)Nandi and Mahakala (to the East);(ii) Herambha and Bhringi (to the South); (iii)Durmukha and Pandura (to the West) and(iv) Sita and Asita (to the North).
Along with the Dvarapalas their subordinates are depicted in minor relief at on the base of the images.
4.3. The pairs of Dvarapalas guarding the temple and placed in its exterior (at the entrances) are larger in size and more ferocious or threatening in appearance , with a "dare not enter" look to their faces and gestures , perhaps to keep away the evil influences. The Dvarapalas flanking the doorway to the sanctum are comparatively modest.
The Dwarapalas in the Hoysala temples are particularly graceful with ornate jewellery to suit the delicately carved interiors; gently holding lotuses as if inviting the devotee to God’s home.
5.1. The historical development in the depictions of Dvarapalas is quite interesting. The Dvarapalas in the Pallava temples were made fierce. But, the Dwarapalas of the Chola temples are truly awesome intended to strike terror in the hearts of the wicked. They are massive towering up on the walls, snarling you down with sharp oversized fangs, riding on the Yali (mythical beast) making one feel tiny and submissive.
However , by the time of Vijayanagar (15-16th century) the Dwarapalas grew a shade smaller but muscular and more ornate; they didn’t appear to lean on a mace or a lance- like weapon but stood tall or cross-legged
5.2. But the artistic excellence in depicting the Dvarapalas reached its zenith in the Hoysala architecture. Their intricate patterns, adornments are chiseled like a jewel, with extreme care. They are magnificent works of art in their own right
6.1. Most of the Dvarapala images are sculpted according to the Agama prescriptions. But the shilpis do tend to improvise and avail artistic liberties. Sometimes, Shilpis the temple architects employed massive Dwarapalas at the entrances to symbolically emphasize the grandeur, majesty and magnificence of the Lord residing in the temple.
For instance, the Dwarapalas at the Brihadeshwara temple of Thanjavur are massive. But, what is more interesting is theme the sculptures devised to drive home the message. The entire Dvarapala panel is basically related to the image of the elephant, the largest land-animal, depicted within its frame; and you have to work back to gain an estimate of the size and power of the Dvarapala.