Unfortunately, the answer is NOT as straight forward as I had hoped.
Lets look at the part number that you supplied as follows:
50000878-03
My source tells me that if the two digit number FOLLOWING the ‘-’ is between 01 and 07 inclusive, then the parts are 2MB FLASH modules. If that number is 08 or above then it could be either (2MB or 4MB FLASH) but is more likely a 4MB FLASH than a 2MB FLASH unit.