virtualbox.org

The current GuestAdditions - on Linux guests at least - only offers 4:3 ratios, as far as I can tell. Ubuntu played nice originally and let me con the system into having widescreen, but now the Auto Resize Guest Display option is disabled after an Ubuntu update so now I'm stuck with 4:3 again. It's a terrible waste for widescreen monitors. Mint and Sabayon both died when I tried to encourage it to give me wider resolutions.

Could we offer some 16:9 or 16:10 resolutions, please?

Use the full screen toggle Host+F.

The guest can only tell what the resolution is that it is given.
This one was taken while in windowed full screen (not host+F)