i'm getting a set of 18x7.5 ET48 wheels and lowering it about an inch... whats the recommended tire size? thank you.

 

215/35/18.

 

Since you have 7.5" wheels you can do 225/35x18 but with the wheel offset you might be close on rubbing on the inside. 215/40x18 will work for sure. 215/35 will work but will give you less sidewall so the ride will be a little rougher and it will be easier to get wheel damage from pot holes, etc.
205/40x18 is 24.46" dia and 3.23" sidewall
215/40x18 is 24.77" dia and 3.39" sidewall
215/35x18 is 23.93" dia and 2.96" sidewall
225/35x18 is 24.20" dia and 3.10" sidewall

I have 18x7" ET52 wheels with 215/40x18 tires on lowered MCS with no rubbing however it is very close to rubbing on the rear inside. I am not exactly sure how the 12mm wider wheel (6mm towards the inside) and 4mm offset (towards the outside) compares with my setup (should be within a couple mm. Thus 215 should work but 225 might not. A small spacer would provide clearance if you want to go to 225.