Try right-alt (or whatever you have selected as the "activation key" and f
(for full screen).

that just makes the host screen bigger, not the desktop.I did not unfortunately.

However, I did figure it out.
It is really stupidly easy: Right click on desktop, Properties, screen
size - increase size. DUH!!! I feel stupid for not trying that now

At least you got it sorted and if you could set a full screen
resolution then the graphics driver must be loaded. One other point
about the OSE version - it doesn't do USB so things like printers and
flash drives cannot be used ( apart from that it's fully functional ).

If you wish to have USB then you'd need to install the full Virtual
Box program from their web site. You'd have to remove the OSE version
first but you'd not lose the virtual OS's already installed - easy
enough to get them back when the full version is installed

Do you have the guest additions installed? You need this to get the most out
of your graphics settings. Once installed then you can change the settings
in the Windows VM, just as you would in Windows by right clicking on the
desktop and changing the settings. Then you can adjust the screen size in
Virtualbox window on the host's drop down menu to match the new screen size.

Did you load the Virtual Box Guest Additions ? This adds in things
like graphics drivers and allows a larger screen size to be set.
Without it you get the basic VGA screen only.