[bigeasy1]

I would think 9x12 tabs would be good for your 18 footer.

The folks at Bennett recommended 12x12 drop fin tabs for my 23 Tsunami.Actually,they said that they would like to see 9x24's on my boat,but because of the Seacraft hull design,it can't be done.
The drop fin tabs,have a 3" drop edge on them,which give much more lift than a standard tab,according to the people at Bennett.

I also have a 21'Trophy and have 9x12 standard tabs on it.They make a ton of difference in the handling dept.I'd be lost without them.

I think any boat,no matter the size,can get big improvements from a set of tabs.