How to assess storage size?

This is a fair question; it’s hard to judge.

Text is essentially zero, to the point you can ignore it.

Images take up more space, but we have knobs available to downscale those and can recompress on the server side if you want to do that.

Videos are comparatively huge and will quickly eat up storage. You should prevent users from uploading them and suggest they use a dedicated video hosting service and link to them.