There isn't a particular weight or size class for requiring hydraulic flight controls. The requirements are for control forces to stay within certain bounds, and in general this becomes more difficult with increasing size and speed of the aircraft.
Keeping the control system manual has the advantage that the aeroforces are fed back to the pilot, and that with increasing airspeed the controls stiffen up, making it harder to making large deflections which would impose high accelerations on the airframe. But larger/faster aircraft require larger control surfaces, with the associated larger control forces. However the control forces must remain within the boundaries of human force ability.
Many of the older hydraulically actuated aircraft have some form of feedback of aircraft velocity. The 737 has q-feel in the pitch channel: an artificial feel system stiffens up the flight control as a function of dynamic pressure as sensed by the pitot tube. The DC-10 and the L-1011 have this as well.
In a previous life I used to contract for simulator modelling of flight control systems of fixed wing and helicopter flight controls, and the Beech 1900 is one of the types that I worked on. It has a fully manual control system, all aeroforces are fed back to the pilot. The pitch channel has a down spring of about 40 lbf, which is apparent from ground measurements. Have heard comments about the pitch forces being high during landing, and that particularly female pilots can have trouble exerting these forces during landings. Hydraulic actuation may make it easier to fly the aircraft and tailor the required forces. There are larger and faster aircraft certified for manual control, the F100 is one: pitch has manual backup.