From my understanding - anything over 17" for stock brakes and stock powerplant is suboptimal for our cars. Even 18" on the rear can detract from your cars performance. Basically - more weight on the driveline means more power/torque required to move the driveline. Lighter is better.
Depends really. For drag racing, smaller is better. 15's will outperform 17's by allowing more sidewall flew to soften a launch. Plus they are usually lighter.
In terns of road handling, larger rims are better to a point. The increase in turning response (due to smaller sidewall) is not always in proportion to the loss of acceleration and braking due to moving the mass of the wheel further away from center.
The heaviest part of the wheel is the rim part whichis the inside of the tire...the metal part. Larger diameter wheels push this further from the hub. It's like the lever theory - the farther from center, the more energy to move.
Now...in terms of looks, it's all subjective to what is proportional to the eye.
For fox bodies, i think 17's look good with the smaller fenders and body size.
For 94-98's, 17's and sometimes 18's look good
For 99+ with larger body and fenders, 18" looks the best. 17's sometimes look puny in the wells.
For 05+ though, 20's look awesome. Once you get awya from the whole bling bling thing, they can look very nice if done right.
There are always exceptions though. I have seen an '04 Cobra lowered on nice 20" rims that did look nice. The problem is most people who put 20" wheels are in the bling bling crowd and put gawdy wheels that belong in a rap video on the car