Set skins using /setskin (url) or /setskin (url) (player)
Note: Url's must directly link the image, they can't just be on the page. Example skin used in gif https://i.imgur.com/mORJxcm.png
Reset skin back to vanilla with /clearskin
Reload skins/clear the cache using /clearskincache
Compatible Skin Sizes As long as they follow the same ratio as the default skins e.g. 64x64 or the old 32x64 then they will be supported. Though the way that minecraft handles textures the numbers will need to be powers of 2. Basically multiply the base size by 2 over and over until you get the size you want.
I have not tried going lower but theoretically that should also work.
New Skins: 64x64,128x128,256x256,512x512,1024x1024,2048x2048,4096x4096,8192x8192
Old Skins: 64x32,128x64,256x128,512x256,1024x512,2048x1024,4096x2048,8192x4096