I currently have 12" Insta-trim tabs on my boat, I've been wanting to move up to 15" in either a small k-plane or billet tabs. I have an opportunity to get a set of 20" K-planes at a great price. Can they be too big? If so, is there a formula or general rule for length of tab to boat size?

thanx

They should be fine. Just mount them about an inch above the bottom of the hull.

Griff,,,i say 1/2 inch is high enuff ,,,1 inch is allot !!!

From everything I have read the longer the tab the better the ride if used correctly.
Dont forget to mount them securely and make sure the transom is up to the task.

I have 28" K planes on a 24' boat. They work great. Don't worry about too big. Too small is more of a problem.