Everything I've found said 19 or 20" due to interference with the suspension. Width is also a concern without the right offset. I've been looking off and on and I cannot find any easy solution to a cheaper square setup.
Maybe if you modified the suspension and/or brakes you could go smaller...
It's fine if you want to risk that, but just know overinflating your tires by doing this is a risk. The reason they get better mileage is due to less tire contact for lower rolling resistance. The risk is not just to blowing a tire, but improper wear patterns and less contact surface so your...
If you have a computer and quite a bit of hard drive space you can download ISTA and it should give you all the instructions. It's a free download if you search around. You will need an ENET OBD2 cable, though. It's what the BMW techs use to diagnose and repair the cars.
You should be leery, you never inflate to that pressure, you always inflate to the pressure specified on the label in the door well. For me it's 33 for front and 41 for rear. The sidewall says 51, which is a manufacturer spec, not one specific for the vehicle. You are likely overinflating...