So I was wondering if anyone knew what the best dimensions for an avatar would be? And what recommendations for a free(cause I'm a literal child and don't have money) mobile app that works for making custom skins do you have? Thanks!

If you have access to a computer at all, I reccomend GIMP. It lets you have layers, transparency, and a lot of useful features if you care to learn them.
No suggestions for mobile, though as im unfamiliar with that territory.

As for avatar size, 100x100 is the maximum but it should shrink images for you regardless (though this will blur the image if it was too big).