24" is enough for field of view if you are sitting 80 cm from the monitor.
28" is great but, pixel density going to be awful, but if you are having trouble to see things well, that might help.
For competitive what you should looking for is a low display lag monitor with a smooth and efficient antighost, crosshairs also will be nice.
However, if you are looking for increase your reaction times you should invest in a better audio, with effective 3D positioning, a good headset gives you more the edge than the fastest monitor. The reason we human beings react 1.5 faster to sound than what what we see.