Factory Subaru wheels are relatively narrow -- 8" or 8.5" on late-model WRX and WRX STI cars. The narrow stock wheel also dictates a relatively modest tire width. A wider wheel means you can also increase the width of the tire.
While you are upgrading wheels, you may want to increase the wheel diameter by an inch or more. A larger diameter wheel means a tire with a lower aspect ratio, and a shorter sidewall. The thinner sidewall can have some drawbacks in decreased ride comfort, and if you live in an area infested with potholes, you may not want to get too extreme. But a shorter sidewall gives more responsive handling on the track, improves the looks of the car, and can allow room for larger brake package.
Our most popular upgrade wheel size for 2015+ WRX and STI is 18x9.5". With the correct offset (+40 or higher is good), you can put a 265/35R18 tire on the car without having to worry about rubbing the suspension or fenders. Most (but not all) of the wheels we sell in that size will clear big brake packages, including the four-piston STi Brembo brakes, and the six-piston Brembos found on the 2018+ WRX STI.
Note that wheel and tire sizing can be a little bit complicated, depending on your needs, your other modifications, and what's available to fit your car. Give us a call if you need help figuring it all out.