58" is what I’m using. I would prefer them to be longer, but it’s what I have and it works. I have a BedRug bed liner which helps, but I’m sure a piece of carpet on the tailgate would do the same.
On my Yakima saddles, I use terry cloth buffer pads. They help it slide nicely and they’ve never blown off.
I guess the best thing about the 58" bars is that I can keep the roof racks in the bed of the truck across the front of the bed when not in use! They don’t take up a whole lot of space that way.