Well any value we choose will be very arbitrary. If we look at it from the point of maximum absolute disk space we allocate for relation at once, the 4MB limit would represent 2.5 orders of magnitude change. That sounds like enough for one release cycle, I think we can further tune it if the need arises in next one. (with my love for round numbers I would have suggested 8MB as that's 3 orders of magnitude, but I am fine with 4MB as well)
I have modified the patch, this contains the max limit on extra pages, 512(4MB) pages is the max limit.
I have measured the performance also and that looks equally good.