I have an 18" ILF riser (basically a B riser), and B ILF limbs. It's 58". The bow is very stable with the larger riser, and the shorter limbs sure shoot fast. I also really like the larger sight window. I have had and A Cheyenne riser, and an A Shawnee riser, too. So far this ILF is my favorite. That being said, I have a 16" ILF riser on order, and C ILF longbow limbs. I should be able to mix and match ad infinidum, as well as have a pretty good poundage range to play with.
Larry is right, get both the A and the B, but don't sell them. If you end up with more than one set of limbs, you're not going to want to swap limbs all the time, are you? :D
Mark