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.
Thanks for your post! Gameplay hitboxes actually come in three sizes for all champs, >small, medium, and large, and don't change based on things like animations, the model, >or character facing. We do this so that you and your opponents get a consistent >experience.
Galio's wing's aren't actually making his hitbox larger, though he does have a "large" >size hitbox already. As warmonkey and Oluldan pointed out, this both helps Galio tank >and is appropriate given the size of his character.
More information about the three different hitboxes can be found here. The three special cases are Nasus, Renekton, and Cho'Gath with their ultimates [Source].
Yes, you are both of course correct, Cho'Gath, Nasus, and Renekton's abilities do cause their hitboxes to increase in size.