Depends how wide your screen is and the overall proportions of your cabinet. The one we make for example, has a 17" screen and two players, and is 16 1/2 inch on the inside edge, and 24" high, so we make the marquee area 5" high (4" viewable accounting for the retainers).

As a guide, make it no more than a quarter of the full height, and keep it similar in proportion to an original marquee's width/height ratio.