The computer just wants the overall circumference of the tire to remain the same as the OE tires. You can install 16 or even 17 inch wheels so long as you adjust the tire size so the overall circumference remains the same
For example, if you were to buy a set of 16" wheels, you could swap from your stock 205/65 R15's to a set of 205/60 R16's and your size would be less than 1% off the stock size and they would work just fine
If you like any of the Ford OEM alloys, you could buy a good used set, or you could buy any quality aftermarket wheel and everything would work just fine. I recently put a set of 2012 Ford Focus alloys on my TC
Don