ok, thanks all for the information so far.
Interesting that there is no easy to find, clear guideline. I'd ike to export from Lightroom in the "right size", avoiding resizeing as well as any issues with forum software etc. Was thinking of 1600 pixel on the long side, which is what I do on other forums usually.
That should work. Since the larger photos get resized as they are uploaded, they all end up working well in the forum. I have not had one rejected for being too large, but it is possible if you have a very large image and file size such that reducing it to 2048 X 2048 results in a file larger than about 5 MP.
I tried making a photo very large by saving it as 9096 X 9096 and came out with a 55 mp image that would not upload. I could probably experiment to find out the limits. I believe that it is not a simple computation. The server itself has file size limits, then the forum software, the ppi, size, and file size all play into the limit.
Then, there are style sheets that control what is displayed and how. I don't expect that many people understand how it all works together, the site coding was probably contracted out.
This may become a issue with high MP camera bodies that have very high density photosites like 100 mp, but as long as the file size is within the limits after the image is resized, it should work. If it becomes a issue, Craig will see if it can be fixed.