I definitely understand what you're saying. You don't want the site to get bogged down with too many large files. It makes sense, in theory, but it also seems like you're implementing a solution to a problem which doesn't exist. I don't think there's a major problem on this site with lots of really big files being uploaded. There absolutely SHOULD be a file size limit, but it needs to be high enough that users don't generally need to worry about it, unless they're doing something unusually extravagant.
It's also worth pointing out that an individual file size limit does not protect you from spammers, because they can just chain post a ton of smaller images. Just today, somebody posted a full 23 page comic all at once
For reference, Comic Fury's file size limit is 3 MB for comic pages, and 6 MB for miscellaneous extra image file upload.