In addition to boat length and displacement, there are three aspects of boat performance that affect which autopilot to select. One is the heaviness of the helm. The second aspect is how fast the helm needs to be adjusted. Downwind, autopilots have to throw the helm over to counteract for overtaking seas. Powerboats need responsive pilots downwind as well, since they tend to slew and yaw as they run down the seas.
Slow pilots get out of synch with the waves, and oversteer as a result.
The third factor is how far the helm has to be turned. Some sailboats hardly notice a 10 degree helm correction, due to their inefficient or undersized rudders, which means that their pilots have to have a long throw. Start with manufacturers recommendations and don't exceed them. Yes, we told you they don't always work, but begin by finding out which autopilot is recommended for your type of boat. If you are near the upper limit of the manufacturer's recommendations, especially in displacement, go up a size.