Too big?

Gonna be getting a tank today for a male Kenyan Sand Boa. Would a 20 long be to big for him?

No, it won't be too big, as long as you are able to keep it warm enough. I regularly kept males in 10 and 20 longs and females in 40 breeders.

That's a good tank for him. Sand boas bury for comfort anyway. Just make sure u get all the essentials. Especially under tank heater and thermostat. Good luck!