The beta theme kind of takes care of this issue. The width is scaled to fit within the page and then you can just click download to see it at full resolution. There's also a lightbox view that'll fit images to screen on both axes.... But that doesn't account for the classic theme where page stretching can become a little troublesome for some.

For the most part I follow your proposed dimensions, sometimes I'll go a little bigger, but if I really want to post a big version I just leave a dropbox link or something to be courteous to classic theme users. I do love me some high res artwork. ^^

Aye, I use the Beta theme too, I think it's much better even if it has its own issues. But as far as I've heard most people still use the old theme.

I myself loves to see detail in artworks, and it never occurred to me that some may find the large sizes annoying until I had a discussion on upload sizes with another artist yesterday.