The official recommendatation are 1280 (width) x 1024 (height).
There are mainly two reasons for recommending 4:3/5:4 aspect ratios:
1. They are available per default, so inexperienced users don’t need to add resolutions to Windows.
2. They provide a few extra pixels for vorpX’s async timewarp.
You can still experiment with other aspect ratios, of course. The performance gain usually isn’t worth the extra hassle campared to 5:4/4:3 though.