Ariel is a figure that defies description, as his magical nature and role in the play makes perfectly clear. For example, he performs most of the jobs that Prospero asks him to do whilst he is invisible. However, at the same time, the play makes it clear that he is a figure who is able to shape shift depending on what needs to be done and the job at hand. For example, he at various stages appears as several flames burning in various locations to a harpy who is able to banich a banquet by beating its wings together. In addition, not only is Ariel able to shape shift, but he is also very flexible in terms of the number of different environments he can operate in. Note that we are told in Act I scene 2 he can: "fly, / To swim, to dive into the fire, to ride / On the curl'd clouds."
The only real reference to Ariel's physical description comes in Act V scene 1 when Ariel performs one of his final enchantments for Prospero. Note what he infers about his size:
Where the bee sucks. there suck I:
In a cowslip's bell I lie;
There I couch when owls do cry.
On the bat's back I do fly
After summer merrily.
Merrily, merrily shall I live now
Under the blossom that hangs on the bough.
Clearly, Ariel is a very small character who is able to ride on the back of a bat or fit inside a flower like a bee. Prospero perhaps supports this impression of his size by refering to him as "delicate" and "dainty" and finally, "my bird." Ariel is a character therefore that we have to imagine in our own mind. As a creature of magic, he defies description apart from the few details we are given in the play.