The "standard" corfam straps (20mm) have a 16mm taper...but if I were you, I would wait until your strap arrives before you buy a buckle. There are many, many different types of corfam straps and you can't guarantee that the one you are buying will definitely have 16mm width.
Try e-mailing Bill (Squinky) who is an expert on all things Corfam (and other types of straps): [email protected]