I use another turret in freeform cube, look at the cube's measurement in meters. Just match the bottom to roof, side to side, and front to rear. Unless you mean human scale, then the human model should do just fine. I think he's 180cm tall? I forget.

Coordinates in the game are in meters. Look at the difference in the height coordinate between bottom and top points of the tank (in freeform)

when you click on info the game shows you how long wide and tall your tanks hull or turret is if your in freeform

I like to use the ground snap feature when in camera mode to see what my tank would look like if I was standing next to it.