People think about leeway in completely the wrong way. Its not really the boat slipping sideways in a parasitic way, its the necessary angle of attack that the boat must make to the water to provide the necessary lift.
Lets make a very over simplified example by ignoring the hull.
Consider a centreboard boat that makes, say, a course made good of 45 degrees to the wind, and in order to do it has the bow pointing at 40 degrees to the wind, which we call 5 degrees of leeway.
Now rotate the centreboard 5 degrees to the hull. The boat now makes zero degrees of leeway. Does it sail at 40 degrees to the wind? It does not. It still makes good a course of 45 degrees, but now the bow is pointing at 45 degrees, not 40. Your actual performance on the water is unchanged. Rotate the board another 5 degrees and you'll make 5 degrees of negative leeway - the bow will be pointing at 50 degrees to the wind, but the boat will still make good a course of 45 degrees.
So, as Tom Speer says, you are really asking the wrong question. The amount of lift from the hull/foil combination will be the same no matter what leeway angle. All else being equal, the bigger the daggerboard you have the less leeway you will make. If you have an asymmetric section daggerboard mounted at an appropriate angle it could even go negative. But much of the effect will be on the angle of the hull to the wind and not on the course made good through the water.
I don't think you're going to get a quick rule of thumb for what size daggerboard will give the greatest improvement in the actual performance of this boat through the water, which is what you should care about. Whether that's with 5 degrees of leeway, 2 degrees of leeway or even -5 degrees really doesn't matter. Its going to depend on dozens of variable to do with rig, hull shape and goodness knows what else. You may also find that in the end the biggest design factors are not to do with performance. How much room are you prepared to sacrifice in the boat for daggerboard cases? How much draft can you have without hitting rocks all the time? But any reasonable size, well profiled daggerboard is going to make an improvement because daggerboards are so much more efficient at generating lift than hulls are.