The size of the monitor doesn't count at all, in any way!
What can be (note: "can be", not "is") important is the resolution you wanna run*. The reason why I say "can be" is because there are cards which would work faster on a specific resolution than other cards with a lower resolution. Or some card work faster with gaming and others with normal apps, etc... It depends on the GPU being used, integrated memory etc etc.
Verte is correct in saying that what matters is your budget. You obviously aim for the biggest/best card you can get for the amount of money you have. That is what matters. All the rest is comparing apples with oranges as each card would be able to control a 20" monitor (or even larger) and most cards (if not all also) can control the high resolution. The question is only what money you can spend....
PS: that's a big monitor... cool!
EDIT:
* Aka: even on a small monitor you could set a high resolution. Thus the size of the monitor does not matter at all... resolution and monitor size are two totally different things.
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