So going BBP with OEM parts widens the front track, I've seen it some where around 3/8" per side. I run a 17x7 with a 19mm offset (4.75 back spacing) and it worked with both OEM and Dr. Diff 13" brakes (with pushed them out another 1/8" per side). But, like 72bluNblu said, you move it too far and you hit the upper at arm with a 14" and 15" wheel. So a 15x7 that has a 4.25 or 4.5 back spacing is ideal but, as I found out are not overly common (especially in 4.5" back spacing) if you want something less mainstream (i.e. Rallye, Steel, Cop, Magnum 500, Cragars, etc) which is why I went 17" custom wheels and got a far better street tire selection at the same or less than 15".
Then there is wheel hub size.
