well we have been here before, first you asked what was the biggest size and then after a lot of advice you still are not sure. so here you go
i have a set of 18" with 225/40/18 on right now. i have lowered the car 35mm with tte springs and have 18x8.5 on all round. i run 5mm spacers on the back so they sit out a bit but there is plenty of room. i run 20" in the summer. 20x8.5 with no problems and have 225/30/20 on there.
so you said in the first post you wanted a wide look for your car and that's why i said run 225 on the back to give it a bit of width and 215 on the front so they are not so hard on you and the steering.
so to answer the question no you wont have to roll the arches unless you are going to sink it like the titanic......low low low. and if your not sure then just stick with originals 215s all round.
i work with tires and can tell you what sizes you can go.... have a little web page that works out the sizes you can run from the size of the original size tires so i guess there is your answer..
check with others about rolling arches and i think you will find that there are not to many that have had to do it...