For belly mounted RAM expansions inside A500, top ceiling is at 1,7Mb, if used with Gary/68K adaptor(Revs3/5/6). That's a 1,7Mb of slow, bogo (ranger) RAM. A500+ could use a 1Mb trapdoor RAM to max-out ChipRAM to 2Mb (Rev8), but then that's it for trapdoor.
IMHO, Alex's proposal might be a best move to play...
