The Commodore REU (RAM Expansion Unit) architecture for the C64/C128 allows for up to 16 MiB - 256 banks of 256 addresses in 256 pages.

Due to the lack of support hardware in the C64 (no hardware RAM bank switching/MMU) this memory is not bank switched and then directly addressable by the CPU, it's copied on request by DMA into actual system RAM. But in some sense, a C64 with a 16 MiB REU is a 6502 with 16 MiB RAM.

But yeah, you want CPU addressable RAM with real bank switching. You couldn't really do 16 MiB, you wouldn't want to bank switch the entire 64 KiB memory space. The Commander X16 (a modern hobbyist 6502 computer) supports up to 2 MiB by having hardware capable of switching 256 banks into an 8 KiB window (2 MiB/256 banks = 8 KiB).

Let's say you design something with 32 KiB pages instead -- that seems kind of plausible, depending on what the system does -- you could then do 256*32 = 8 MiB and still have 32 KiB of non-paged memory space available. I think this looks like just about the maximum you would want to do without the code or hardware getting too hairy.