Well when you go to change your avatar you'll see some text below the upload button that says something like "maximum of 150×100 or 3.5 MB (whichever is smaller)"
So basically, even if the avatar is 150×100, if it's larger than the allowed file size then it won't play.
I had this issue with my waluigi avatar that says "you suck" (aka one of the greatest avatars I've ever had lol) where its file size was too big so I had to reduce the quality and even trim it to about 105×100 so that the GIF would play as my avatar. There are online websites that are really helpful in resizing/cropping GIF files. If you're still having issues then I could help in cropping or fixing the GIF you want to use.
I hope that helps ^o^