In my personal experience, a 22″ monitor was rather small (it’s hard to get immersed when the whites of the walls are still visible) and a 40″ flat-screen television was far too overwhelming (you need to move around your head to see everything).  After testing numerous monitors, in my personal experience, I would that 25″-30″ is ideal, filling the range of vision without exceeding it.  The current market is still dominated by 20″-24″ monitors, so for the time, bigger is better.  Paying extra for 2 more inches sounds trivial, but those inches are applied across the whole screen and the whole immersion factor is increased overall.

Resolution plays a part too.  If only using 1080p, you may notice some distortion at larger sizes.  However, with 1440p (QHD) or 2160p (4K), distortion won’t be a problem with a larger monitor.  Especially, if using 4K, you will want to go as large as you can afford to.

ASUS VG248QE 24" Full HD 1920x1080 144Hz 1ms HDMI Gaming Monitor
ASUS 1ms Gaming Monitor