I am a proponent of reasonable size motors in sailboats but often deal with owners who want to put much bigger motors in their boats than I recommend. My 36ft boat had 20hp, my 34 had 12hp and my 38 footer had 18hp. That 12hp could push my boat against a 40 knot wind on flat water. Sure, it was slow progress and the motor was working very hard but it could do it. In less extreme conditions the motor wasn't just ticking over to move her at reasonable speed. A 20hp motor would still be acceptable on that boat but anything bigger would be over-powering it.
Professional Boatbuilder magazine has an enlightening article on this subject on their website, written by their technical editor Steve D'Antonio. Steve also works with owners and builders through his own business, Steve D'Antonio Marine Consulting, Inc.
Please read Steve's article. And for info on my designs, please visit http://dixdesign.com.