Isn't it similar when you got Ubuntu as guest?
No, it isn't. And it has been a few months since I installed Ubuntu 10.04 and went through this. I thought that before all I did was install the VirtualBox extensions, bump the graphics memory from 12MB to 64MB, resize the host window to the desired size, and then the new window size would show up as one of the options under Monitors | Resolution, but that didn't do the trick. Guess I'll need to google it again.