Ha Whitley
I see you have the 20 footer. That would dictate the size taps you will need. If you look at the pictures of the i/o and where they placed the tabs that is where you want to place your tabs. Seacrafts are a longitudinal step hull and on the 23 footers that last step outboard is approx 15 inches thus a 12 x 12 is really about your max for the boat. Adding drop fins traps more water under the tab and increases the lifting potential for heaver weights on the stern like a stern drive of twin outboards. On my 23 with one engine the 12 x 12 with no drop fins work fine. I could probably even go with the M 120 which are a 12 x 10 and a 1 inch drop fin.
You will have to measure the last longitudinal step on your boat hull first. As our Bennett web sigh says you really want to measure in approx 3 inches from the end of your transom and get your width from there to where the next step starts going down again. That will give you your span for your trim tabs I would go with 12 inch chord to give you a bigger bite on the water. With out measurements my guess you will need the M 120 for you 20 footer or maybe a 12 x 9 tab.
Here is another page for our sizing guidelines
http://www.bennetttrimtabs.com/size1.htm
You can contact me at Bennett just ask for Ken McGovern after you measure that last step if you would like to talk.
Here is a picture of my transom as well.
FellowShip
Just for the Grins