I don't think that monitor is capable of standard res (CGA), which makes it next to useless for most arcade games. It's NOT the normal switchable CGA/EGA that Konami loved to use around that time, and I know some other games on the hardware Keyboard Mania ran on from that timeframe were in fact VGA, so it probably is a VGA monitor.
I don't think the seller knows much more about it.
If it follows the same numbering scheme as other Toshiba monitors from that timeframe, it is in fact a 29" (27" USA) monitor. They tend to be pretty nice, but if it's VGA only, you won't be running 99% of games in native resolution on it.