Apologies for the delay, it took a while before I had time to pull the wheels off and check them.
From the looks of it, the rears are marked with 126 55, and the fronts are marked with 124 51, I'm assuming that's what gives the wheels the different sizes?
I now have the wheels back in the original configuration, but now I'm wondering if this is actually a valid setup for this car in the first place ( previous owner put these wheels on ), and maybe this is the root of my steering wheel shake?