In most arcade businesses today the game operator usually populates the arcade with a mix of screen sizes depending on the game/cabinet.
Countertop games usually have a 13 to 14 inch monitor.
Older classic games had a 19 or 20 inch monitor.
1990's era games started using 25" monitors.
Crusin' series sitdown drive games use a 25" monitor.
San Francisco Rush sitdown drive games use a 27" monitor.
Some new games and "showcase" cabinets use 33" or 39" monitor.
Super deluxe games like "Let's Go Jungle" use a 62" DLP projection tv set (which accounts for the high price tag of this machine!)
So you see there is no average screen size in today's modern arcade business. Most arcades also have a mix of redemption games, pinball machines, pool tables, dollar bill changers, air hockey, jukebox, etc and they don't even use a monitor in those.