If it’s saying image size, that means you are trying to paste in the actual image instead of linking to it.
Most gifs are hosted elsewhere (like giphy) and hyperlinked. Make sure you are grabbing the image address.
Is this what you are asking?
A custom gif, one you made, needs to be <1mb to upload. We pay for space, so we need a limit. I am evaluating compressing/archiving/purge of older uploaded images due to server space.