I'm making edits in my community to reduce disk size on the server, removing junk and editing the image upload max sizes. But wounding what's a good size. At the moment, for most members, I have 2MB max image size that they can upload and for upgraded and staff accounts 5MB max upload size.
Other sites that I'm a member on you can upload almost 10MB images size, and they are just gaming related images LOL
Long run, if images are 2MB each and you have members uploading 500 a day, that tends to add up in total size. I have large storage anyway, but that's not the point. So far my site is 6GB in total and my weekly backup does 20GB max