whatever the stated maximum X by Y resolution is.... this is the maximum the monitor will support. as waffle stated software will not allow you to change this.
however, you CAN downgrade 720p/1080i/1080p signals down to 480p (or other custom resolution) through software. this is counterproductive though as any gains through a high-res source are lost.
if you truely want 720p/1080i/1080p video you will have to invest in a monitor capable of displaying them. the prices for such monitors aren't bad at all.