You should play around with a calculator:
4x4 Excel Spresheets
There's not just a golden rule to it. placement of the links are going to change your travel, pinion angle, and anti-squat. The longer length you go, the less pinion angle change you'll get so you could potentially have more travel. You also get really heavy because long tubes will deflect, and you have to combat that with diameter and wall thickness. You'll also start crashing into the frame and cross-members.
I'd just recommend taking a couple weeks with that calculator before ordering up steel!