So I went with the sizes of 245/45-19 and 275/40-19 for the front and rear respectively. The rims are 8.5" and 9" in width respectively. This allows for the up-size to be a little bit "meaty" as some might say. I'm OKAY with this as this will give me a tiny bit more "curb-protection" when someone (not me...
) will drive the vehicle. The X3 is already a fairly compact car and putting bigger wheels on the unit would be utilizing the wheel-wells to the max. From what I found, the rears were the tightest from any control arm or shock with a good clearance of 1/4" from any critical component interference.