Thanks to all who replied. Well, I did what I should have done in the first place: I called the company. The guy told me that the reason the one piece recurves are shorter is that those are the most popular sizes for that style. I guess I can blame market forces for this one, though I don't see how it would be much trouble for them to make more lengths available. Anyhow, he also told me that it would be no problem to modify the plans, just add the additional length in the limbs near the riser. When ordering supplies, I should inform them of my modifications and they would be able to determine what lamination thickness/taper I would need.