How is the bandwidth of an actuator selected for a primary flight control surface?
Actuator bandwidth requirements typically flow down from guidance documents like MIL-STD-1797 (Flying Qualities of Piloted Aircraft). Essentially, actuator bandwidth is selected to ensure the flight control system has a sufficient stability margin. For example, see p. 235 of the above referenced document where the bandwidth of the open-loop response from pilot command to control effectors has to fulfill a minimum gain and phase margin.

In particular, this paragraph is relevant:


  A measure of the handling qualities of an aircraft is its stability margin when operated in a closed-loop compensatory tracking task. We refer to the maximum frequency at which such closed-loop tracking can take place without threatening stability as bandwidth ($\omega_{\mathrm{BW}}$). It follows that aircraft capable of operating at a large enough value of bandwidth will have superior performance when regulating against disturbances. A bandwidth criterion is especially useful for highly augmented aircraft in which the response characteristics are non-classical in form (i.e., have large mismatch in equivalent system fits).


In any case, there are other reasons for actuator selection of which bandwidth is a major one. For example, here's the rationale for actuator selection for the A380 flight control system. The selected electrohydrostatic actuators have a higher actuator bandwidth versus the conventional hydraulic actuators fulfilling the stability margin requirements with the potential for improved disturbance rejection.

Higher actuator bandwidth enables the potential for even better handling quality, e.g., gust load alleviation, however that would need to be assessed in relation to actuator lifetime reliability and overall power requirements.