In the last few releases I've found VirtualBox likes to be set up with a screen size that is some of its standard recommended sizes, and if you choose something oddball (such as dragging it to some unusual size) you get that tiny screen punishment when you resume a saved-state machine.
It is claimed on the VirtualBox forums that this is a regression in 5.0.4 that persists right through 5.0.12. IDK.
Some people have luck with the Automatic screen size setting. That's been hit or miss for me.
You want to also give it all the graphic ram it wants or some sizes are simply unavailable.