Ubuntu will only allow 1024x768 or 800x600 resolution options for my external monitor, an HP 2159m (which supports 1920x1080). The laptop's built-in monitor works fine. I'm running Ubuntu 12.04, and I'm new to Ubuntu.
I've check the HP site, but they only have Windows drivers for this monitor. If it's any help, my laptop also has Windows 8 installed and the external monitor works correctly in 1920x1080 with the generic windows driver.
Any help appreciated! Just let me know if you need more info.