It's more complicated than fore/aft. The boards/rudders work in conjunction to provide the necessary lift (lateral resistance) to make the boat go forward.

At low hull speeds, the amount of lift created per area of foil is less than at speed (related as the square of the velocity, just like drag). Therefore at lower speeds, you need more board to provide the necessary lift to "stop" lateral slippage.

The only thing that moving boards forward or aft does (with conventional straight foils) is change the relative center of lift of the foils in relationship to CE/CL of the sails. You don't create additional lift, you only move it. i.e. if you move them further forward, you will load the rudders more and change the helm.

With regards to length, typically the designers are going for efficiency. The aspect ratio of the board is directly related to their efficiency. Longer boards are more efficient, but don't produce as much lift at low speeds and have a MUCH narrower grove. Short board provide more lift at low speed, however are less efficient, but have a wider grove.

One other factor is "tripping". Longer boards will fly a hull faster than shorter boards b/c of their increased lever against the hulls.

It's all a trade off for one performance gain against another. Build something that is theoretically more efficient, but won't ever hit it's range and it's likely to be slower than a less efficient platform that will be able to reach it's operational range.