Pick what diameter you want, I used 1 1/4", and go. I've had bars as long as 60" and they really aren't necessary. I used to like the long bar for setting up both side, getting lengths correct. Really don't need it. So I went to a 48" bar which is plenty long enough do to one side up to about 63" housings.
Shipping was nearly as much as the metal itself.
If you have a hydraulic cylinder that went bad, that's another recycling spot/use for an alignment bar.
Send me a PM with an e-mail address, I can get you an excel spread sheet that I did that computes tube and axle lengths for Dana60 and 8.75 applications.