What GPU do you have?
Size will depend on your layout and distance, as will resolution (although GPU plays a big factor here).
If you're looking for 1080p and sit a reasonable distance away, 24-27" is probably fine.
If you're up close, you'll probably notice pixels on a 27" @ 1080p.
Once you start looking at 1440p, 27-32" is probably a good range.
There are other factors though - refresh rate, panel type etc. Size is just one component.