I think the problem you describe isn't one:
When a user uploads a large image using the editor's photo upload it will land in the original size in their profile's album. But by default the photo upload offers to insert the image in "Large" size. And that's actually the largest possible image size. Users cannot even select "Original" as size to insert the picture. Screenshot of image sizes available from editor's photo uploader So from a 3MB uploaded image, the actual embedded image file which could impact your page performance gets down to 141KB.
So I'm all with Jason here: Allow as big as image size as you can, the UX will prevent them to be too large in size.
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