Hi, I have only good experiences with running obenbsd inside virtual box. Last time I used openbsd like this is about 1 year ago. Performance was good. Screen resize via randr possible ( I had to define the video modes on the virtual box cmd line, beware of the color depth). I don't know what exactly you are trying to do, but playing 3d games inside the virtual box openbsd guest might become difficult.
x11, Rdp ( virtual box include or as application inside the guest) is also a possible solution to connect to the VM via network an run virtual box headless. This might give you another option for screen resizing. Jan
