There's no such thing as "too big for coding".
That being said, your constraints are usually desk size and price. You generally want the most screen for at the best combination of these.
Dell apparently has some pretty good large monitors for a good price. MaximumPC has some reviews that I think found NEC devices were quite good as well. I'll look up some links shortly.
UPDATES more info and linky goodness:
Higher resolution will mean things on the screen are generally smaller. You may want to visit an electronics store and look at pages of text on various screens and see if any of them make your eyes swim more than others.
MaximumPC's Best of the Best has a 27inch running 1080p for $300, which is amazing. They also really like a 24" Dell right now, which is also for around $300. And if money is no object, their favorite is a 30" HP for more than the Apple screen. :)
UPDATE pure opinion:
I have never been wealthy enough to buy the screens I want to buy, so unfortunately (fortunately?) I must rely on the opinions of MaximumPC which I have generally found trustworthy.
Personally, I'd probably prefer one large widescreen and one smaller "square" screen, just because that's how I think and I wouldn't need any special screen management tools to get windows where I wanted them. I want multiple screens up and visible at any given time, and will generally prefer to have windows maximized in one or both of them. So I'd put my IDE in the big screen and leave SO or whatever reference site or email client or itunes or secondary interest on the smaller screen for quick reference without having to minimize the big "work" window.
That is entirely me though, and may be very difference depending on if you're a left- or right-brain, artistic or not, multi- or uni-tasking, linear or non-. I'm left- and right-brained, artistic, multi-tasking, and very, very non-linear (which makes it somewhat of a mystery to me why I find end user IT support so rewarding) and so being able to leave a focused and important thing on one screen and just leave it for discipline sake is important to me while also feeding my need to have multiple sensory inputs, referential material, etc, on that other screen for the quick fixes of info or data or whatever.