It depends on what I'm doing it for and how long I'm willing to wait. I'll probably start out with a 640 x 480 in Bryce to get a rough idea. If I'm doing it for the screen, it'll go up to 1024 x 768 for a final render. If it's for print, I'll kick up the largest dimension to 3000 or so for 300dpi images that will fit on a typical piece of paper. If it's something simple for a web interface or icon (usually buttons or whatever), then I'll actually go smaller. Probably with the largest dimension around 400 or so. I'd probably try more fancy stuff (at decent resolutions too), but I still don't have the computer for it yet (at least with my level of patience). Thus I tend to do more modeling than rendering.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.