The load a "Flying Stab" puts on the servo is usually much smaller than the load you would get form the same size aircraft at the same speeds with a fixed horizontal and having the elevator hinged off the back end.

since you have a VERY low speed airplane and assuming a passably balanced pivot line (25 to 27% MAC of the moving surface is the ideal range for the pivot line. I can't tell for sure about yours being in that range... without knowing the shape...) You should be able to use a "suprisingly" small servo. One of the sub-micros should do it.

I have had a flying stab type model that had appx 15 sq in stab area, pivoted on the 25% MAC line of the stab, and the plane was capableof in excess of 140 mph. I used a 27 in-oz servo, and had plenty of authority at any speed.

With your lower speed... you will be in a LOWER required power range than I was... if that stab is pivoted on the correct line.

if you have it pivoted too far forward (with sweep... its possible with the described dimensions...) you might need as high as 40 in-oz...

Which specific aircraft are you doing? if its a home design... got a picture? (put a stripe of blue or tan masking tape on at the pivot line so we can see where it pivots for sure...)