Not big
A six horsepower engine will be plenty big for the Daysailor; you could probably even use a four horse engine with no problems. I use an eight horse on my Oday 23, and it pushes the boat at hull speed in calm water at about 3/4 throttle. Most longshaft models are a standard 20" (regular shaft models are 15"), although I seem to remember that at least one manufacturer offered a 25" model. Go with the longest shaft you can find; you will appreciate it in a chop or powering through wakes. If you decide on a four stroke engine, make sure your mounting bracket is specifically rated to handle one; it ain't just the additional weight you need to worry about. Four strokes generate significantly more torque than two stroke engines and need a beefier mount. If you go with a four stroke engine, I would say that four horses would be plenty.