Monitor for Macbook Pro Retina Display

I want to purchase a 13 inch Macbook Pro with a retina display and a monitor but lack the physical desktop space for the 27 inch Thunderbolt monitor. Assuming I want a monitor of equal or greater quality to the Apple monitor, what are some options with monitors that have a smaller footprint (right now I have a 21 inch display that fits well, although I could go somewhat larger).