The reason I use the 15" monitors is because of my lighting desk. On a 15" monitor, the playbacks line up perfectly with the faders on my Chamsys PC Wing. Having standardized touchscreens across my laser and lighting setups means I can mix and match in a pinch. I'm yet to think 'Oh, I could do with a bigger monitor' for either application. I find that I tend to hang mty hand on the top of the monitor bezel and hit cues with my thumb. On a 15" I can reach most bits of the screen like that. Probably not on a 17", and definately not on a 19"+.