Re: Kicker motor
What you should be concerned with is not the size but the features. The 9.9 for example is a purpose built kicker with an over-sized lower geared lower unit that is longer than the standard unit.
This gives you... DC charging of 6 amps to run as much DC power items as you like without fear of killing your battery's (thats a big bonus), a stronger lower unit that will last much longer, a deeper bite into water below your keel, the ability to use a prop style (they come standard with it new) that is more suitable for slow powerful and better control. These things are really big in everyday use, others will work but not like one purposely made to do the job.
I have the Yamaha 9.9 and its awesome, its on a 26 footer and has plenty of power, but it would work just as well on a 16 foot boat. It uses about 1 gal of fuel about every three hours so it paid for itself very quickly.