You actually want the furler extrusion to be shorter than the forestay, as long as the extrusion is longer than the jib luff. You will potentially need to be able to slide the furler up the forestay to access the forestay turnbuckle for adjustment. That extrusion (whether plastic or metal) can usually be shortened to fit, but if plastic... can not be made longer. The metal (usually aluminum) extrusions come in about 4' lengths, so you might be able to purchase an additional piece from the manufacturer if the used furler was too short.
Do you have the DWYER mast or the Z-SPAR mast? I have the standing rigging specs for the Z-SPAR mast (Thanks Joe!, late, great owner of TRINKA)