Re: What size should the kicker be?
I can only relate my personal experience, so I hope some others will chime in.
The kicker was on my "nice to have" list and I was waiting for a deal on a lightly used motor with a long shaft. They don't pop up that often around here, so it took awhile. If you're buying new you can tailor the motor to your boat better.
I was tempted to buy a 10-horse, but when the Merc came along at a good price I decided to go with it. It seems to have enough oomph to push the boat in just about anything I'm likely to encounter. I guess that's my recommendation - go as big as you can without paying too big a weight penalty.
8-10 hp is probably enough for trolling. If you want to make sure you can push the boat along against heavier seas, wind and current, I'd look in the 15-20 hp range.