I would keep the blitz units at kinda medium size (if it is a game where you control a human, you use 1 blitz unit = 1 meter, if it's a flightsimulator, then you say 1 blitz unit = 1/10 kilometer for example). I guess the maximum x,y,z coordinates is the maximum size of a float (cuz x,y, and z coordinates are float values), but I can't remember how big floats can be again right now in my head.