on an '87 gxl what are the largest rims i can get for them? can i get 18's or is 17's the biggest that'll fit? and what would the width be 7"?

 

https://www.rx7club.com/suspension-wheels-tires-brakes-20/official-fc-wheel-fitment-thread-257700/

Read that

You can get 18's, but they look too big IMHO and will only hurt your performance because of extra weight and increased moment of inertia. They'll also cost more, as will the tires.

I think 17's look really good on the FC, and that's about as big as one should go, my oppinion.

I'd max out on width, and practically speaking that means 8" wide wheels and 225 tires. 17x8's with 225/45/17's is a good combo, and that's a very common tire size, with lots of high performance tire choices. For an 8" wheel you'll want an offset that's around 30-35mm. Too low a number and it'll stick out of the fenders and rub, too high and it'll contact the suspension and rub. Neither is good.

Mustang GT wheels are a very good budget wheel, as they're reasonably light, they're strong, and they're cheap. They also fit just right. That's what I run on my car.