[GeorgiaSnake]

Depends on what car and axle you have.

Measure from outside of the hub to outside of hub.

Measure distance from the hub to body. Double the hub to body dim add the axle width. Subtract 1 1/2 to 2" for tire clearance. Now measure the distance from your balls to the floor no wait that's for something else.
Divide by 2 and you have the approximate outer offset. Measure from the hub to the inside of the body or whatever is closer to the hub add that to the offset and you have the approximate width.

If that doesn't work I can probably come up with a more difficult set of instructions.