currently, the pics aren't being resized on upload (although this is a feature we're planning to implement in future versions). This means: if you allow a user to upload 3 MB per pic, he's allowed to do so, and coppermine will only resize the intermediate pics and the thumbnail to the size you set in coppermine config. This might soo lead to you running out of webspace. The alternative is to reduce the individual file size the user is allowed to upload, but this would require the user to be capable to size down large pics (that come out of modern digital cameras) to decent sizes (that work on internet pages), but alas most "regular" users don't have the skills to do so.
There's one workaround for users running Windows XP: they can use the XP_Publisher tool to upload, which will resize the pics on the client without the user noticing it (the drawback of course is that not every user has Windows XP).
This being said, it's hard to give a general advice on what file size you should limit your users: if they are expereinced (e.g. if your site is dedicated to webmasters), you should set the allowed file size low (e.g. 200 Kb). If they are unexperienced (and using XP_Publisher is not an option for you), you should allow high file sizes (e.g. 2 MB per pic), but then you (as an admin) should regularly use the feature "Delete original size photos" from the admin tools to delete the big full-size pics your users upload, leaving only the intermediates in place.
Hope I have made my point and my posting is helpfull for you.
GauGau