I'd do 6" carbides. I have a 1.25" track on my sled, I have the precision skis on my sled, and run 4" bars all the way around, and it's pretty good. If I ever wear out the carbides that are on there, I have 6"/4.5" shaper bars to try next (A friend of mine works at Stud-Boy).
Friends of mine that have 1.25" tracks on their sleds with 4, 6, and 8" carbides. It's all just a matter of what works well for you. No one specific length is going to work for everyone.