Since you are working with pre-existing wheel arch flares, you are aesthetically constrained to remain within the tire diameter limits dictated by the radius of the wheel openings of those fender flares. The second consideration shall be to keep the chassis level (parallel with the ground) without having to raise or drop either end of the car excessively. Group 4 flares are usually designed for a large difference between front and rear tire diameters, similar to the standard body versions. The GT5 and GT5-S flares are designed for only 1/2" difference between front and rear tire diameters.
Once you have the tire diameters figured out, you can begin looking at tire widths available in those diameters.
You'll find the best front tire selection in 17" or 18" wheel diameter, but you'll also find that as the tire diameter goes below 25" the availability of tires dwindles. The width of 17" rear wheels is limited by clearance with the lower rear control arm. You'll definitely want at least an 18" diameter wheel in the rear so that wheel width is not limited by this clearance issue. As for rear tires there are choices in both 18" and 19" diameter wheels, but tire choices are more plentiful with the 19" wheels.