@RWillieK:
I thought DPI only made sense for printers.

For monitor display what makes sense is pixels, so in that way the maximum size is already defined.
What's important here is
filesize, whether it's a 200x200 pixel image or 450x450, it's makes little diference.
Saving images on your server and publishing links in the thread, even though it saves bandwidth, has been disencouraged. This happens sooner than later: some of those images will get deleted, leaving the threads relevance hanging on pictures that cease to exist on their original servers. Publishing these pictures on the forum, guaranties that they'll be all in one place, allowing better management.