Ok.. Pixies hmm....
*precosious-fairy-teachers-lore-mode-activated*
Striktly speaking, a Pixie or Pixy is a Devonshire fairy, more or less the same as Puck.
Alternate spellings, dialects and variants of Puck include Pisca, Poake, Pouk, Pouke, Pucke, Puckle, Pug, Púka, Pukis, Pukje, Pwca and many more. The Irish pooka, or púca, and the Welsh pwcca usually refer to similar spirits.
Since the thing is basically an Irish myth, exact pronunciation is hard to come by. In the end Puck & Pixie go down to the same myth (even though modern interpretations of the two don't have much in common).
Either way, the Puck (Pixie) is an extremely mischievous nature or household fairy in English/Irish lore. It's mostly known for playing spiteful tricks on unsuspecting humans, which leads to often embarrassing situations, but in some cases its been known to champion the poor and oppressed.
Favorite tricks include: changing shapes, misleading travelers at night, spoiling milk, frightening young girls, and tripping venerable old dames.
Descriptions of him range from a hobgoblin to a fairy, brownie, goblin, an elf or a thing more or less that of our beloved D&D Pixie.
A Puck (Pixie) can, as has been said, shapeshift and some say that this is a confirmation of his tricky personality, often associated with shape shifting abilities.
Most people, though, think of him as having a hairy body, with goat feet, like a satyr or faun, since that way he was portrait in Shakespeare's A Midsummer Night's Dream and in Kipling's Puck of Pooks Hill. Far and wide the most loved and renowned versions.
D&D (or some earlier game/novel??) has made the Pixie an altogether different beast I assume, but the original Puck (Pixie) would IMO likely be small (in D&D terms), about the size of a human child.