Start by doing some measurements.
You have a center line of reference- the surface where your existing wheel rides on the axle. Call it the prime meridian...
Start by measuring that point to the inner fender surface.
Then from the same surface to just inside the outer fender.
Add those two to get the overall distance from just inside the outer fender to the inner fender surface.
You'll have what you need to get guidance from and wheel and tire pro.