Couple things to know about external monitors:
Resolution - how many pixels can fit on your screen. So 1080p = 1920x1080, 2560x1600 = 4.096 million pixels, etc.
Aspect ratio: the ratio of pixel width and pixel height. 1920x1080 has an aspect ratio of 16:9, 2560x1600 has 16:10 (8:5 if you wanna get mathematical), and the old 1280x1024 has 4:3.
PPI (pixels per inch) - how densely packed each pixels are on the screen. So if it's a 1920x1080 27" monitor, it would be (sqrt(1920^2+1080^2))/27 = 81, which is pathetically low for a monitor. At 2560x1440 on a 27" screen you get 108, and 2560x1600 gets 112. You can see how big the difference is between 1080p and 1440p or 1600p.
For future reference, the hi-res screen on a MacBook Pro 15" is around 128.6 ppi.
Color gamut - how many colors your display can show. Yes, the screen you're staring at cannot display all of the colors of the rainbow (at least not accurately enough). A 100% NTSC gamut screen can show all colors of the NTSC spectrum. Here, a high color gamut is best, but it won't matter as much if you're not into graphics. A screen calibrator will improve any display's colors.
TN/IPS, LED, etc. - technology behind the screen. TN is twisted nematic. Cheap, mass produced, less accurate colors. IPS is in-plane switching. Has better colors and viewing angles. LED backlit displays usually draw less power and have truer whites.
Contrast - how big the difference is between blacks and whites. You probably know how this works.
Brightness - self explanatory. However, a bright display usually corresponds to a one with low contrast.
Refresh rate - how fast your display works. 60Hz means it can display 60 frames per second, and 120Hz can display 120 fps.
Wattage - how much power your display draws. Lower, the better (obviously

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Verdict: Either 27" display with 2560x1444/1600 resolution, or 21" with 1920x1080 resolution. 60Hz, good contrast & brightness, IPS if possible.