Go for 27" if it is 2560x1440, otherwise go for 24".
Don't go an just buy a cheap one. Research it. I don't know if you are like me, but I've only ever bought flat screen 4 monitors - Over the past 10 years I've gone from a generica 17", then to a Dell 22", then a Iiyama 24" now I'm on a dell 30".
The Dell 30" was far an away the best investment I'll ever make, and I'll not replace it unless something higher resolution comes out.
I waster a couple hundred quid with the cheap 1080P Iiyama monitor. White Light bleeding around the edge of the bevel is AWFUL - It's worst of the monitors I've ever had.
Find the monitors that meet your needs and then post back and we can help you go through and pick the best...