I'm not familliar with the HS 81, but in general, I would say that you won't need anything bigger than a standard servo (1 for ailerons and 1 for flaps)

Noiw I'll probably get blasted for saying that, but I've flown 60 size planes with one standard servo for ailerons for MANY years. The newest "Trend" is for going to a 2 servo setup, and it may be nice, but it's not a necessity.