I put a 20" LCD in my Taito cabinet that I am MAMEing. It originally had a 19" arcade monitor mounted vertically, my LCD is mounted horizontally. I play lots of vertical games on it and it is a good size in that cabinet. The games are displayed at about 14", so they are slightly smaller, but you do not notice it. That size monitor is good for a small cabinet like the Taito which is a 1 player cabinet.
What size cabinet are you putting it in?