I did run into a problems with IceWM and VB, it seems the menu bar on top of the VM window would go off the screen and I couldn't bring it back down.
When I tried it with JWM, it worked great.
If you mean you couldn't resize the window due to its being partially offscreen, try this:
Click on the Restore button (between Minimize and Close on the top right). Then hold down the Alt key while you click and hold the left mouse button and drag the window so you can get to an edge to resize. With most programs, you can resize a too-big window so that it fits your screen.
Some programs are not cooperative when it comes to resizing. I don't know if VirtualBox fits in this category.