I haven't had a lot of time to play with this, but I'm trying to find an all-around standard size to process images to post here.
Viewing on my iPad that looks pretty sharp to me
When I open the original and the resized versions on my desktop to the equivalent of a large iPad screen there is barely a quality difference. I sized this 2000 mp on the long side and saved at just under 2 MB. I think that will be my universal size for uploading going forward. I was spoiled for years being able to upload full size images on DPR but I fully understand the server resources that would require of a new site. I really appreciate what the guys have done here in such a short time.
IMO, it's just a waste of space and bandwidth to make things too large. I'd stick with 2048 or so in the long dimension, or a bit more, and moderate compression. I rarely find a need to post anything larger than about 1 meg.