Tom,
If you did that you would reduce one form of drag at the expense of another. Basically the drag due to lift would drop while the drag due to wetted surface would go up.
There can be some benefits by increasing the length of the board provided you don't increase area. But as mentioned earlier these benefits can come at a price.

BTW:- There is no limit to ratio/size of dagger boards in the rules.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!