What was the size and resolution of the monitor you tried?
There are many large displays 24"-27" that are only 1920x1080 in resolution, making the pixels significantly larger than the ones used in your internal display. If you are limiting yourself to a 1920x1080 HD display, you will want the smallest size available to have a similar pixel size to that in your MBP. A large screen at that resolution might work ok for video oriented projects, but for text and detail based projects, the difference between your internal display and the external can be noticeable.
Over the weekend, my old external display died and I ordered a 22" 1920x1080 display at a fair price. The pixels should be just a bit larger than my internal display, and it should serve me well. Unfortunately a larger high res monitor was not in my budget.