How should the maximum output force of the actuator be chosen?
If the largest hinge moment is known for the entire flight envelope, including that required for continued safe flight and landing after failure(s), then the actuator can be readily sized. Of course, some engineering judgement is needed to compromise between:


Buffering for additional unforeseen loading (i.e. factor of safety) for later stages of the design and flight test, during which swapping the actuators can be costly, and
Sizing down the actuators to save weight and costs.


Therefore, the more scenarios you have considered for obtaining this maximum hinge moment and the more confidence you have in your model, the better off you are. For example, for elevator, consider the following (but not limited to):


Achieve -1g to 2.5g at Vmo/Mmo with 3 sec out-of-trim in the opposite direction (14 CFR 25.255(a) and (c))
Achieve 1.5g at Vdf/Mdf with 3 sec out-of-trim in the opposite direction (14 CFR 25.255(a) and (f))
Takeoff rotation with dual hydraulic failure
Achieve 0.8g and 1.3g at Vmo/Mmo with dual hydraulic failure (or single elevator surface jam)