What are the various physical appearances of Rakshasas?
Citing from The Mahabharata, Book 7: Drona Parva: Ghatotkacha-badha Parva : SECTION CLXXV, describing the battle between Vikartana's son, Karna, and the Rakshasa Ghatotkacha.
Of blood-red eyes, Ghatotkacha was of gigantic form. His face was of the hue of copper. His belly was low and sunken. The bristles on his body all pointed upwards. His head was green. His ears were like arrows. His cheek-bones were high. His mouth was large, extending from ear to ear. His teeth were keen, and four of these were high and pointed. His tongue and lips were very long and of a coppery hue. His brows were long-extending. His nose was thick. His body was blue, and neck red. Tall as a hill, he was terrible to behold. Of gigantic frame, gigantic arms, and gigantic head, he was endued with great might. Ugly and of hard limbs, the hair on his head was tied upwards in a frightful shape. His hips were large and his navel was deep. Of gigantic frame, the circumference of his body, however, was not great. The ornaments on his arms were proportionate. Possessed of great powers of illusion, he was decked also in Angadas. He wore a cuirass on his breast like a circle of fire on the breast of a mountain. On his head was a bright and beautiful diadem made of gold, with every part proportionate and beautiful, and looking like an arch. His ear-rings were bright as the morning sun, and his garlands were made of gold and exceedingly bright. He had on his body a gigantic armour of brass of great effulgence.
A Rakshasa of terrible eyes, fiery mouth, and blazing ear-rings, acted as his driver, holding the reins, bright as the rays of the sun, of his steeds in battle.
Once more beholding that foremost of warriors, viz., Ghatotkacha, fearlessly approaching him, surrounded by a large number of Rakshasas that resembled lions and tigers and infuriated elephants in prowess, some riding on elephants, some on cars, and some on horseback, all armed with diverse weapons and clad in diverse kinds of mail and diverse kinds of ornaments;
Ghatotkacha, that prince of Rakshasas, who was endued with terrible energy and strength, and who, inflamed with rage, then looked like Yama himself. From his eyes, as he was excited with wrath, flames of fire seemed to emit, like blazing drops of oil from a couple of burning brands.
Soon the Rakshasa seemed to be like a porcupine with quills erect on his body.
History of the Devil: Brahmanism and Hinduism describes them as follows:
Such are the general mischief-makers, the Asuras, the Pretas, or ghosts, the Bhutas, or spook-spirits, the baby-killing Grahas, the Rakshasas, who are either giants or vampires, not to mention all the other demons of less power and importance.
Description of Shuurpanakha in araNya kANDa is as follows:
She that demoness who is facially unpleasant one with that pleasant faced one, pot bellied one with the slim-waisted one, wry-eyed one with the broad-eyed one, coppery-haired one with the neatly tressed one, ugly featured one with the charming featured one, brassy voiced one with the gentle voiced one, deplorably oldish one with the youngish one, crooked talker with the pleasant talker, ill-mannered one with the well-mannered one, uncouth one with couth, abominable one with amiable Rama spoke, besieged by Love-god.
One thing we can definitely conclude is that Rakshasas were ugly looking with huge body structure, long unkempt hair!