To choose your monitor size in my opinion,
depends fully from your distance to the monitor.
This has something to do with the monitor resolution so that you won't see pixels.
For me sitting with my eyes around 3 feet away from the monitor,
for 24", the resolution should go not lower than 1080p
for 27", the resolution should go not lower than 1440p
Just remember you need stronger hardware for games on higher resolution, while keeping at least 50fps.
@ Mugglensu1984:
40" on 1080p at 3 feet away sucks!
But on 4k, now you are talking!
The thing is, your HW however must be quite strong for that.