VirtualBox machine window is suddenly very small

I am running Ubuntu 16.04, and have an Ubuntu 16.04 VM running in VirtualBox. Today when I started up VirtualBox and started the VM, it didnt resize to fit my display. Instead it is stuck at 800×600, and there seems to be no way of increasing it. Auto-resize does nothing, resizing til 1920×1200 does nothing…

I even tried reinstalling VirtualBox, as I had to to do it once earlier because of some weird error. It worked fine after the first reinstall, but now suddenly the VM screen refuses to go above 800×600.

Thanks.