If you look around you will see a LOT of 9.9 HP engines on sailboats with outboards in 25-26 foot range. That alone should tell you a lot. Also in Long Shaft versions at your local marine shop you will find a lot of availability in this horsepower range.
The Pearson 26 is just over 5000 lbs. At the 2hp per 1000 then 10 hp is perfect.
Many areas also have regulations regarding vessels over 30 feet or 10hp and over. Therefore a lot of small boats and sailboats use 9.9
Finally weight is the biggest consideration other than HP. Too heavy sinks the stern and too much power may put excessive stresses on the bracket/transom or whatever the engine is mounted on.
Our last boat was a Hinterhoeller Niagara 26 displacing just over 4000 lbs and 26' 8" LOA. This boat was originally powered with Honda 4 stk 7.5 horsepower from the factory. Lots of power except when motoring into 35 - 40 knot gale in shallow water with close huge waves. When thinking of replacement my choice was the Yamaha high thrust 8 hp which had only one negative - $3000+ in the configuration I wanted.
Friends have an Aloha 26 displacing also 5000+ lbs and possibly a bit beamier than the Pearson. They use a Honda 15 hp. However it is important to remember that th Honda 15 and Honda 9.9 from that period were the same engine with the same weight (well a minor difference in that 9.9 was detuned 15 or something like that).
Our current boat is a J27 - 3800lbs also narrow beam. It came with a 4hp short shaft which we traded for a 4.5 hp long shaft. Is plenty of engine in flat water but when the wind and waves come up not enough power for my liking as we lose a knot or more of speed. Last season the carb. needed cleaning and we switched to a borrowed brand new Johnson 9.9. The stern was too low, the engine too much weight and power for the bracket and all that throttling up did was dig in the stern and not provide more than 1/2 to 1 knot of speed. I really did not like this engine on the boat but it sure beat walking. My choice would be a 6hp if it weighed no more than the 4.5
Sorry for the long post. 9.9 hp is all you will need or want and is less expensive than an 18. The 18 may even damage your boat