If stability sizes the tail, the stabilator and conventional tails would be the same size.  If having enough control force from the tail is what determines its size, the stabilator can be a little smaller.  If you are converting an existing airplane with conventional tail to stabilator, I'd keep it the same size and shape, but give the stabilator less movement.  Be sure to hinge the stabilator forward of the quarter chord.

I used Wild Bill's design articles a lot.  One of them was very useful in sizing an elevator or the position of the pivot on a stabilator to get a useable pushrod force.  If you use the area of an existing stabilzer/elevator and hinge it at the front the controls won't be able top move it effectively.  A pivot with 10-22% of the area in front of the hinge will work, but it was handy to use his calculations to make sure the bellcrank would move it properly(i.e. not run out of control into the wind, overhead).