If you have a gap between your baseboards with a duct opening in between, then you have a situation that requires a baseboard register. Unlike typical registers, the duct size isn't all that important. The important thing is the width of the gap between the baseboards and what overall height you need to cover everything that needs to be covered. Before calling in, please make not of whether the duct opening is on the wall, the floor, . . . or both (it happens). Also, check and see if the metal ducting is sticking out from the wall. Sometimes that can cause problems. Whatever the case we'll talk about it.