not sure of the weight of your boat, but my Pearson 26 has a Merc 15hp 2-stroke. Pearson recommends 10hp max in the docs but my friend the prev. owner went with the 15hp for when extra power is helpful to punch through. It is useful now & then.
Definitely go with a long shaft, ours is a transom mount 20" shaft and I was stopped by cavitation last year in choppy 4' waves into a 25mph breeze, couldn't keep the prop in the water. I may look for a 25" shaft next time if I can find one.
Next time I might go with a 9.9hp 4-stroke, though it weighs the same as our 15hp two-stroke.
Yeah, 6 to 10 ought to be plenty for a T/4 - I've used an old 9.9 HP Honda 4-stroke 20" shaft on my old San Juan 24 (3600# boat) which seemed a bit too much, both power and especially weight (almost 90#?!) - like the guy that said half throttle is all he used.
Also used a new Tohatsu/Nissan 6 HP 4-stroke 25" (55#) shaft on the San Juan 24 as well as a little on my more recent Cal T/4. Even with the 25" there are still times in rough chop when the engine rises out of the water and "screams", which I hear is NOT the thing.
I'm very happy with the 6 HP, power-wise, and will note here that its built on the same "family" of engine block and other engine parts as the 4 and 5 HP (at the same weight) - different cam is the main difference, I hear. Just as the old honda 7.5 HP is made on the same block and other parts as their 9.9...
So, I'd always buy the 4-stroke, always the largest HP for a given block "family" & weight, and I'd ALWAYS buy the 25" shaft length for transom-hung sailing applications, vs the 20" (don't even conside the 'short' 15" shaft length for sailboats, except for no-wind, mill-pond-smooth conditions).
Hope that helps.