a 19" screen is a common size for alot of classics...it really depends on the style of cabinet you are looking for. if you try to put a 19" monitor in a cabinet designed for a bigger one, it can look silly. I had a 19" LCD in a dynamo cabinet that was designed for a 25" monitor..it looked really stupid.
But i currently have a 19" LCD in my primary 2player mame cabinet, i'm not sure what the cabinet was originally, but it did have a 19" monitor, so it fits well. I also have a 19" CRT in my Donkey Kong, which is what DK originally had, so it works fine.