Thats cause the offset is higher on the BRZ. closer to 50+ while the legacy offset is around 45+ i think? idk im not sure.
Find a set of used wheels if your looking. You can easily find a great deal if your patient
As for size, you can stay with a 17" or go to an 18" but get something with a 35+ offset.
My Legacy on XXR 527's 18"x8.75 +35 offset