I am only guessing here as I have never owned or been on a 226 (except for boat shows), but I suspect a 15-1/4" x 17" would be a good starting point. I would go to both the Grady site and click on performance specs for your boat and visit the Yamaha site and do the same. If you are still unsure, contact Grady via email or telephone and they can send you test results with the same or similar setup.
Some things that may make my guess a little off would be what weight do you typically carry (full fuel with aux tank or single tank 1/2 full), do you fish rivers and back bays by yourself or do you and three other big guys head offhshore often?
I'm sure others with better information with regard to your setup will chime in.