I have a Dell laptop with a resolution of 1600 x 900 and I have a second monitor as Dell 24". The resolution that Ubuntu is configured in a virtual box is 1024 * 800. How can I fix the resolution on the Dell monitor to get a higher resolution?
It's most likely a limitation of your virtual machine's drivers, or at least the default installed ones. Check your video settings for the virtual machine. You may be able to try a different "video card." Also, make sure "Enable 3D acceleration" is enabled in the virtual machine settings. Make sure to restart your virtual machine after any settings changes.
Additionally, you can try checking for drivers from within Ubuntu. Simply make sure apt-get is up to date, and check the "Additional Drivers" application to see if either have detected video drivers.