245 /45 R18 square fits easily, I have those on my front. A 275/40R 18 will fit in the rear.
My GUESS is you could do a 255/40r18 square without issue. And likely 255/35r19 as well, given you ran spacers.
I like my 18s for added comfort, but its still pretty crashy on our terrible roads. The biggest trick would be new bumpstops (many e90 guys run e46 m3 fronts/ z4 rear bumps)
Check out this thread:
https://e84.xbimmers.com/forums/show....php?t=1312450