As it stands Porsche Taycan wheel sizes will range from 20-22" with bigger wheels out back depending on model.
If this sounds familiar to you that's because it is...
The 911, Panamera and other Porsche's have a similar wheel size layout and should be among many typical Porsche offerings carried over to production Taycan's. The only confirmation of this ahead of the 2019 launch, is what we can be seen in spy shots and there are many! A couple examples posted below show two sides of the coin, one being (what I believe) is a Taycan "Turbo" sedan with the sportier wheels and big brakes (and Sport Package styling kit?). The other, a Taycan Cross Turismo (wagon/estate) with aero wheels.
Porsche has often been generous with wheel options so I expect the same to play out here as well.
What has been common in coverage of Taycan top-tier trim test mules/prototypes are 21-inch wheels in front and 22-inch wheels out back. In others showing entry-level versions its 20-inch sized wheels with tire specs around 275/40R20. I strongly believe those 275 wide tires make it to production. Why? 911's start with 245's and don't have as much power/torque to tame. Likewise on the higher end where 295's are one of the widest in the 911, seeing 315's like the Panamera will be necessary.
The Taycan sits between the 911 and Panamera so its reasonable to suggest shared sizing and even tires, if Porsche isn't developing something specifically for the Taycan.
If you care about having a comfortable ride, sticking with the smaller 20's to allow for a thicker tire profile might just help. Some reports of Taycan prototypes suggest it can be harsh and unrefined on local roads. I hope that's not the case in production versions but if it is, going for the 21-22" size wheels will come with a noticeable trade off. Maybe over time the Taycan's 115.0" wheelbase, chassis and suspension components can be further refined to handle rougher road conditions.
Taycan Turbo Wheels?
Taycan Aero Wheels?