Custom Axles to allow the use of non m e46 rear trailing arms and brakes

I know suggesting going to a smaller brake size sounds idiotic but whats the smallest size wheel we can currently fit on the rear of a e46 M3? 17" maybe a specially made 16"? the only thing required to bolt up the non m rear trailing arm/ brake setup would be an axle that had the outer cv joint/splines that would fit into the non m hub. anyone have any suggestions on who to contact for custom axles? I have everything else needed.