My forks are 48" since any fork frame I've seen that will fit my loader comes with 48" forks unless it's custom ordered. Longer forks make it easier to catch part of the pallet and scoot it toward you if it's in the very middle of a car hauler trailer where you have to load/unload from the end and don't have enough reach to fully seat the forks under the pallet on the first try. A pickup bed is shorter than that so I doubt I'd have had any issues if my forks were 6" shorter. Worst case, just pop the tailgate off and you just bought yourself about two extra feet of reach.
The 48" forks poking out a little beyond the far side of a standard pallet hasn't been an issue either. Even when putting a pallet right against a wall, setting the pallet down and repositioning the forks a little before "final placement" will fix that issue.
I'd really base the fork length on the size of the tractor that will handle them. A 1-3 series unit would do best with a set of light duty 42" forks, a 5 or 6 series will get a much heavier set of 48" forks, and a 4 series could use either.
