I see misinformation on this far too often.
The WordPress screenshot is shown at 387×290. But, the recommended size is 880×660. This information is freely available on the WordPress Codex.
Why 880×660? Because certain devices have high pixel density displays (HiDPI) which, in a nutshell, will show more pixels in the same space to provide a crisper image. By making your WordPress theme screenshot 880×660 you are allowing for these high pixel density displays.