If any phone is going to be subject to the same force at the edges, regardless of length, then that's what you want to be measuring. It doesn't matter if a long phone can handle slightly more torque than a short one, if it's going to be subject to a lot more torque.

I don't know what the stresses applied to a phone in a pocket actually look like, but it's not obvious to me that measuring force rather than torque is unfair.