In theory, it would be possible to implement the bank select register with a 74HC377 octal D latch, allowing up to 512 kilobytes of PRG ROM, but no Rare game used this much memory.
With 7 bits for indexing banks you could go as high as 4MBytes, so where did the 512KBytes figure came from? Is that because of the position of the mirroring bit (there's only 1 unused bit between the bank index and the mirroring bit)? That wouldn't stop you from using all 7 bits, you'd just get a strange layout for them (or you could even bump the mirroring bit to the highest bit).
Also, the mention of Rare is pretty random, and even looks like something tepples would do here in the forums, but in the Wiki it's just plain confusing. Why would Rare be the only developers that would make large games?