How to determine the maximum servo torque required for a nose gear?
Assuming that we are designing a steering system, for a single steering wheel aircraft.

The servo is yawing the wheel to a position. The position causes side force on the wheel. And that generates a yawing moment for the aircraft. (The aircraft can be moving in various directions, but assume you know the max side force required.)

This side force, multiplied with the distance of the wheel contact point and the wheel yawing axis is one major contributor to the servo torque. (Torque1)

Another contributor is the rotational friction of the wheel. Which is the resistance to yawing further. (Torque2)

Torque_total = torque1 + torque2.

This is the output torque for the servo mechanism. The mechanism will affect the torque on the servo shaft.