On what wheels?
45 will likely cause rubbing, especially if you're lowered even slightly.
255 will fit fine on the back (assuming stock wheels?) but I wouldn't go that wide in the front. Might want to ask this in the wheel and tire forum to get more help.
The most popular tire sizes for 19s are 265/30 rear and 235/35 front. But it depends on the wheels you're putting them on, of course.
2008 E90 335i // Black Everything // photos
COBB tuned / QTP e-cutouts / Helix FMIC / cp-e DCI / AWE BOVs / Black M6 wheels + Toyo Proxes
(and yes, I am a girl)