It wouldn't require a Physical x16 slot because that would mean it has12-15 unused traces. If it only uses an x4 or x1 interface it will never use and x16 because the rest of the contacts would be useless.
For example, in my x16 slot, if i wanted to change it into and x8 slot, then i would simply cover the rear contacts in that slot. I believe in a PCI-E comparison, a review site did such a thing.
As long as you have an x1 or x4 available you will be fine, i would venture to say that they will never use an x16 slot; they simply dont need that much bandwidth (Nor is any manf going to include 3x x16 slots on one motherboard.
-Kevin