Like someone said, it depends on the distance from the screen. Thus depends on how you are going to have it setup. A larger display in a showcase cabinet that you will be further away from may work well; but in a regular cabinet, may result in issues. I've been told by a local guy that sells a lot of cabinets that people always buy a 25" over a 19" when given the option; however, he told me he would prefer the 19" for the games he plays because he can see everything on screen without having to move your head all around.
One of my favorite games is Robotron. A 19" is great. I would not want a larger display in a regular cab to play it because it would make it impossible to take in the entire screen at any given time. Larger for a showcase cabinet would be OK as it would be further away.