Use a 14x6 on your 91 four stroke. Standards work well on all but the rudder.

On mine I had a 24" extension for each elevator and rudder servo, and used twin elevator servo mixing on my 9C. Each aileron servo had a 6" extension, with a 6" extension plugged into each aileron channel in the receiver. I used the flaperon mixing on the 9C. Hope this helps.