You could get away with a 15" TFT, but it would be best to go for a 17" to really appreciate it.
Check out the reviews on this site for what is good and what is not.
Make sure you understand about dead pixles before you buy so you won't be disapointed should you have one or two dead pixles that aren't enough to qualifiy as a fault.
Also check out how much movement you get in the screen. They often have very limited adjustment in terms of height, and this may not suit all users.
Pay a visit to PC World or similar even if just to get a feel for what one is like.