1 unit = 1 cm. 100 units - 1 meter. And so on.
Default speed value is 600 cm/sec which is equal to 21,6 km/h - an average run speed IRL.
Ideal values is the values that you find ideal for your game. Doom guy run with about 60 km/h, however in some games you can’t run at all. So do what you want to do and you’ll be fine.