I have the guest additions installed. I've increased the resolution to (1600x1200 max). My monitor is a 25" (widescreen) and supports up to 1920x1280. How do I get the window to resize to fill the entire screen. It seems like Ubuntu only supports 4:3 ratio and not 16:9. Is there a way to fix this?
I'm assuming you have tried Full Screen and Seemless modes. My guess is you might not have to increase the Video Memory you have configured for Virtual Box.
For size estimates: with 64 Mb of Video memory configured in Virtual Box, I am running a Full Screen Mode display of Ubuntu 10.04 at 2560x1024 resolution.