My brother is an engineer and he has 2 HUGE monitors, like 28" or something crazy like that. My husband is a programmer and he complains his 20" is too small.
So, yes, the bigger the better. Try
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TigerDirect.com - Computers, Computer Parts, Computer Components & Electronics
You can buy any brand - most LCD panels are made by the same 2 or 3 manufacturers and then they stamp different labels on them. I do have to say though, if he is picky about technology it would be best if you let him pick it out. There are little details like inputs and outputs and contrast ratio and typically the return policy on hardware isn't very good (restocking fees, etc.) so you might want to keep that in mind.