I have a 27" (US - 29" Japanese, 68cm Europe) monitor in an MK2, and it feels a fine size, but I'm a bit used to somewhat more modern games than most people on here consider "classics". MK2 itself is hardly a "classic" by most definitions, but it was designed for a 25" monitor, and the 27" barely fits. You can tell it's bigger, especially if you've played the same cab on a 25" recently, but it doesn't feel out of place at all.