Will
17x7
5x100
48mm offset
fit a stock 2001 Legacy GT?
and what size tires would i go with?

Thanks

 

Those will fit, but they might be closer to the fender than you'd want. I have 17x7.5" +48 Rotas and they're perfect, so that .25" may not make much of a difference. Perhaps someone else is running that width on their GT and can let you know.

Keep the tire as close in overall diameter as stock, for reasons of speedo accuracy. Google for a wheel size calculator, like http://www.miata.net/garage/tirecalc.html , for help in finding something as close as possible.