If you are referring to the 2MB upload limit:
There is no user-friendly way to change it. It's set to 2MB in AvatarValidator.
An extension could extend that class and then use a container binding to override it.
If this is done, the webserver configuration might also need to be updated to allow larger files.
This would have no effect on the file size of the final avatar, since Flarum always resizes and compresses images before using them as avatar.
If you are referring to the width/height:
This value is also hard-coded to 100px in AvatarUploader. There is also no way to change it without overriding the full class via a container binding.