I've never seen or heard of this error - nor can I find it with google.
As long as the new memory fitted properly in the slot, it should be the correct type. Is the new larger memory size recognised by Windows? - if so then evertthing is hunky-dory.
On some systems I have seen the message Mem size error when adding RAM - on the next reboot, the new memory size has ben logged into the BIOS and the error doesn't appear again (I think Compaq systems do something like this). Could this be what you saw?