I have purchased Feral Warwick skin at the last discount. When I select it, Warwick looks bigger to my eye. Is it because of colors or is Warwick actually bigger? Most important of all, if it is bigger am I easier to target?
Short answer: Skins do not affect the hitbox of champions.
There are only 3 hitbox sizes in League of Legends, and skins do not change which hitbox a character uses according to this post.
Kennen is a small hitbox. Ashe uses a medium hitbox. Cho' Gath uses the large hitbox.