I voted < 500kB because that's a pretty safe maximum. There are very few cases where you should ever need more size than that in a single image. Average probably around 100k.
If you can't say it in 800x600 or such*, should you really be saying it at all? Consider first of all, cropping, brightening and sharpening your image. Second, if you must show detail, why not crop sections, or take macro shots? There is so much more information you can present, in so much better ways.
*This was once a common screen resolution, but that's largely of historical significance anymore, even among mobile devices (which have vertical or mixed aspect ratios, oddball numbers, and, some of them have bizarre cutouts in the display?...well, anyway...

). Anyway, similar number of pixels, about a half a megapixel, really.
Most people, simply give it no thought whatsoever, have no clue that this is even a thing to argue about -- they have no need or care of the details, they just want to get an image from here to there.
For their benefit, there could perhaps be an added step like, "Would you like your image to be reduced automatically?" The default option would be "yes", encouraging its use. Answering "no" might give a second nag, "would you like your image compressed for faster loading?", which would save it at JPG compression 80-90 say, or PNG indexed, whichever is smaller; assuming this isn't a tremendous load on the server of course**; and if declined both times, just let it through absolutely normal. Possibly add an option "Use these options for all attachments?" for when a user is posting a lot of images and doesn't want the nag every time. Or put it in the user profile as another option ("attachments: advanced mode" say?).
**It could be done clientside with a JS library, though I'm not sure quite how slow that would go. Which... oh neat, Canvas.toDataURL takes a type ("image/png" (default) or "image/jpeg" for example), with a second parameter for quality. So it's probably in most browsers to begin with, no page overhead, and it should be very fast.
But, speaking of laziness, it may well be more effort to introduce such a feature to the forum software, than it is to provide for the server and connections, and if most users aren't complaining about load times or poorly formatted images who cares, right?
Tim