Disassembling my Z-120 today, I saw it has a 85-2653-1 5Mhz model main board. From what I understand, this should have 192K total in the three memory banks. I know a later 8Mhz board model had 768K on it.
These are TMS4164-20NLJ chip, which are listed as 64kbit / 8KB chips if I read things...