What can one conclude about Puck's physical description in Shakespeare's A Midsummer Night's Dream ?
Shakespeare usually relies on indirect characterization rather than direct, meaning that he does not come right out and tell us what a character is like or looks like. Instead, he paints the pictures of characterization through dialogue and through other characters' responses to the characters. Puck is one character that is characterized indirectly.
One thing we learn through dialogue about Puck's appearance is that he is very small. We learn about his size when Puck describes one of his antics as hiding in an old woman's ale mug pretending to be a crabapple. When the woman takes a drink, he then spills the ale all over her neck, as we see in his lines:
And sometime lurk I in a gossip's bowl
In very likeness of a roasted crab,
And, when she drinks, against her lips I bob,
And on her withered dewlap pour the ale. (II.i.48-51)
Since Puck is imitating a crabapple, which was used to spice drinks, we know that he is very small.
Another thing we can surmise about Puck's appearance is that he may actually be very ugly. A Puck is a Hobgoblin, which is not only a devilish sprite, or fairy, it is also a goblin, which is a particularly "grotesque," or ugly sprite or elf (Collins English Dictionary).
Finally, we also know that Puck has very speedy flying capabilities. We know this because, when Oberon commands him to find the "love-in-idleness flower," Puck replies, "I'll put a girdle round about the earth / In forty minutes" (178-179). In other words, Puck is saying that he'll circle around the earth in forty minutes.
Puck is usually portrayed as small, quick, and impish. He is described as a "sprite" by the fairy he meets in act 2, scene 1, which means he is a fairy or elf. In the same scene, he describes himself as being essentially Oberon's jester -- "I jest to Oberon, and make him smile" -- and while it is up to any given director to decide what that means for his appearance, a very common interpretation is to cast Puck as quirky and high-energy.
We know that Puck can move very fast, because he says that he will run around the world in 40 minutes to find an herb that Oberon has requested ("I'll put a girdle round about the earth/In forty minutes," act 2, scene 1). While this doesn't explicitly tell us his physical appearance, it does support the popular impression that he is small and fast.
We do know, however, that Puck can change his appearance at will. He says in the same scene:
"And sometime lurk I in a gossip's bowl
In very likeness of a roasted crab..."
"In very likeness" means "looking exactly like," so Puck is able to shape-shift.