SpaceEngine's GPU memory requirements
are highly dependent on the display resolution. SE needs some areas in the memory called "render targets" to which it renders the image. Their size depends on number of pixels on you screen. Some numbers (on default SE settings):
1280*720 - 525 MB
1920*1080 - 825 MB
2560*1440 - 1248 MB
3840*2160 - 1701 MB
Enabling MSAA anti-aliasing raises up memory requirements. Terrain engine requires 2 GB max, and other systems about 1 GB. So GPU with 4 GB of video memory will be perfect for 1920*1080 screen, but at 4k screen (3840*2160) terrain engine will reduce its cache size, which will limit maximum LOD you can use. For geed experience on 4k screen you need at least 6 GB GPU.
The poll is limited by 10 points, so I can't include all possible resolutions to it (and this is not important anyway, I just need the rough statistics). So please choose the option which is most close to your screen resolution.
As SE does not support native multi-screen, the only way is to run it on multi-monitor setup is by stretching its window across all screens. This is reasonable only on a setup of multiple identical displays, so mixed resolution setups are not included to the poll. Most likely you will run SE on you primary screen (as I do), so choose it.