Best wheels IMO are 15 x 7's on Mk3 fiestas. Running 15's means your handling/geometry isn't effected too much, and your speedo is still accurate. Anything bigger upsets the handling characteristics of the car.

I personally run 15 x 6.5's with 45's tyres and a 40mm drop. With 15x7's and 50 tyres i had major rubbing problems. Now it only rubs very slightly on the inner arch at full lock.

HTH

Cheers