I use a 5hp 1999 Nissan 4 stroke on my 18.5' O'day 192. I had a 2hp 2 stroke on the old 15' Harpoon daysailer. Planning to use oars on the 14' 300 pound GP14.
I could probably get by with a 3.5 hp on the O'day. Sage Marine uses a 3.5hp on the Sage 17, runs at hull speed far under WOT.
If you only ever use the motor to get home in no-wind situations, go smaller, if it's less weight. But probably the 3.5, 4, 5, 6, 8 from Tohatsu/Nissan/Mercury are all the same 1 cylinder powerhead, just with differently tuned carbs, so they probably all weigh the same. I think it's when you hit that 9.9 range that they start using a 2 cylinder powerhead, and the weight will increase significantly.
I know folks with 2hp 2 strokes that do just fine with a Precision 18. Lots of people putting around with the Honda 2.2, but they are air cooled and REALLY noisy compared to a water cooled. There's this Precision 185 I know it's coming when I hear that characteristic Honda air cooled sound. I'm always amazed at how fast that 185 goes with that 2.2hp...
And I know people with Torqeedos that are fine on Precision 18, Com-Pac 19, and O'day 222s.
A couple guys with O'day Daysailors and Mariner 2+2s are happy with the Lehr propane outboards. Lehr should have a long shaft out by now...