I’ve just fitted some stainless aftermarket pipes to my car (from Simply Performance) – these are 1.8″ /45mm diameter.
Whilst there is generally plenty of room around these pipes over and around the drive shafts and through the suspension subframe, they run very close to the bleed nipples on the inboard rear calipers, so I would say these are about as big as possible – unless you repostion the pipes to give clearance which puts them at unsuitable angles for the connections at each end. Remember that these pipes are hanging on brackets at the top of the curve over the axle, so there is no much scope for repositioning them.
I can’t recall now if these were larger than the originals, but the originals had greater clearance to the bleed nipples, although this could be down to the form of the pipe.
In conclusion, I would say its not so much an issue of pipe diameter, more down to the shape of the pipe.
Hope this helps, Norman.