Here's a link to Bennett's size recomendations chart, (scroll down).
http://www.bennetttrimtabs.com/size1.htm
Personally, I don't agree with their recomendations. I've had the standard 9 x 12's on a 204 WA, a 23 Gulfstream, and a 24 Offshore. They all performed fine... plenty of trim.
The newer sport tabs use a different design that achieves the same resultls as larger tabs. Imo, the M80's, (8 x 10) are fine for a 20 footer, but there have been some people who have used the M120's, (10 x 12). The sport tabs also feature an easier way of connecting the tubing to the actuators. It's done from the outside of the transom. This is especially nice, if you don't have room in the bilge/battery area to make the typical elbow connection that's used on the standard tabs. You can check out their installation video -
http://www.bennetttrimtabs.com/movie.htm
Since you should have access from inside, the standard 9 x 12's would be fine for your 209. I also recomend, (if you decide to go with the standard tabs) you order the model that has the rocker switch's and not the joystick control. The joystick is a joke.
I do like the space saving design, and the easier installation of the Sport tabs. I haven't personally used them, but I've only heard good reviews about them. I believe the rocker switch is standard on the sport tabs.
Here's some links from where I ordered my last tabs.
Standard tabs, (includes rocker switch) -
http://www.starmarinedepot.com/Bennett+Trim+Tabs.html
M80 Sport tabs -
http://www.starmarinedepot.com/Bennett+ ... +Tabs.html