I think the taper is done to desensitize the roll rate and to also have less unsupported flap area / mass out where they can get buzzed by the dirty air at the wing tips.
For a plane this size I like the inboard chord of the flaps to be 1 3/4" and the outboard to be about an inch.
I glue hardwood end caps on both ends of each flap to help eliminate cupping, and inbed hardwood dowel chunks to support the control horn hardware