I don't see a "boost tab" on the rudder there. The overhang at the front isn't a tab, it's a balance horn.
A true boost tab is hinged to the surface, but driven by a pushrod connected to the structure ahead of the surface. The moment arm of the tab provides an aerodynamic assist to the surface deflection.
They are generally fairly small because they are effective.
Most frequently seen on late pre-hydraulic controlled planes, such as the T-6.
I've seen 4 servos paralleled on rudders in the larger scale models.
A recent magazine cover showed one such with 6 on ailerons, and 4 on the elevators. With probably another 4 on the rudder, and one for the motor, @ $150 each... I'll never have THAT airplane!
