There's a web page here: Car Bibles : The Wheel and Tyre Bible
which will tell you what combination of rim diameter and aspect ratio - which is the sidewall's depth expressed as a percentage of the tread's sectional width, IE a factory spec bB tyre is a 185/65%
x15", the 185/55% ones that some use are too small and a 195/60% is close enough to be legal in terms of odometer error. Then, when you go up an inch on the wheel, the aspect ratio must be reduced proportionally, which is where the calculator tool on that web page comes into its own.
I don't know what margin of error is permitted under USA construction & use legislation, but here in the UK, 10% under is permitted on the circumference of the tread, but not so much as a fraction of a percent larger is allowed, as that makes the odometer and speedometer read low, which could cause involuntary breaches of the speed limits. Slightly smaller circumference is ok though, as that will cause the recording instruments to read high, so making the driver think that they're driving faster than they are.
It's unlikely that an 18" wheel would be technically
legal here as even the lowest aspect ratio ("profile" as the GTA generation will insist on calling the percentage) available for an 18" wheel may be enough to take the total size in terms of circumference and overall gearing higher than these numbers are with a stock 185/65% on a factory wheel.
So go as large as will fit under the mudguards, but if the circumference of the new wheel/tyre combination is too large, then allow some extra cash for having the speedometer/odometer transponder replaced with a bespoke one to set the dials' readings right.