I've speculated that the available wheel sizes will be:
20" x 8.0" Front
20" x 9.5" Rear
or
18" x 8.0" Both
The rationale is in this thread:
Standard Wheel/Tire Size for Model 3
I would be surprised if they were anything but this. However, this doesn't help you until you have the offset(s). I'm guessing you won't have those until after the initial deliveries and owners take off the wheels to inspect the inner spokes or wherever they're marked.
Does Tesla even list the offsets for S/X wheels now? The offsets aren't shown in the online store.
Also, don't you also need confirmation of the lug size/spacing? It's a good bet that those are same as the S/X wheels, but that would be an expensive mistake.