Personally, I usually have pixel dimensions (longest) between 1000 and 1500, although keeping in mind that I also usually post 2:1 or even 3:1 aspect ratio, so the longer than usual dimension doesn't create a bigger than expected file size.
As expected, there is always a compromise. Again - personally - I try not to exceed 500kb for an image posted here or on my website, so in terms of JPEG quality, I'll stick with 12 if it's < 500kb, but will reduce to 11 or 10 if need be (which appears to still be "visually lossless" to me).
Probably a good time to mention that - again personally (so not official CiC policy), I don't like images to get to the 1MB mark unless there's a good reason for it; in isolation it's OK, but even with ADSL speeds it can still take a few seconds to load which again is OK in isolation, but it can start to bog down the loading of competition threads ... and heaven-help anyone still on dial up!
Having just said all that, I do think that many folks post images here at too low a resolution when wanting C&C; in my opinion, images with a longest dimension of only 600 (or even 700) don't allow a quality evaluation of focus & sharpening.
So the best compromise in my opinion (no surprises here) is the way I do it; aim for around 900 x 600 or slightly above - and aim for 500kb max (and reduce quality from 12 to 11 or to 10 to achieve) (but not below).