In my case, I currently have a 24'' 1080p and I find it quite good (except for some artifacts caused by the Anti-glare coating, but that's a different story).
I've also used a glossy 18.4'' 1080p monitor, and I found it to be really enjoyable, even more than mine. Image was a lot sharper, both for the image quality istelf and for the increased dpi. Still, it was a bit small, especially windows letters, so I would recommend 21'' or 24''.
I don't recommend a 27'' 1080p.