19" was the standard for many (but not all) classic games. Starting around 1990, the 25" size started to become standard then 27". Alot of game operators are complaing that the newer & bigger monitors add so much weight to the games that their moving crews are getting injured (bad backs) because of the heavy games!
As to having room to rotate a monitor, it all depends on the particluar cabinet and monitor. Remember how the corner of Gorf's monitor stuck out the back door and it had that black plastic protector?