The stock hardware has 8MB of flash memory.
It should be possible to expand the NSLU2 to a maximum of 16MB of flash by replacing the 64Mbit Intel StrataFlash flash chip with a compatible 128Mbit one.
The IXP420 has 24 address lines on the expansion bus and the flash also has 24 address lines. Unfortunately the 24th address line on the IXP420 is not connected to the flash at all. This limits us to a maximum of 128Mbit.
A23 (pin 30) for the flash is connected to R64 and EX_ADDR_23 (ball J24) on the IXP420. By removing R64 will make it possible to support a 128Mbit part. R64 is located between the flash and the RTC battery.
A24 (pin 56) for the flash is pulled (high?) by R78 and isn't connected to the IXP420 at all which limits us to the before mentioned 128Mbit part.
RedBoot and the kernel will need to be modified to take the extra flash in account.