The CRC check does not use the size of the file. It sums each byte of the file in some manner. There are many different techniques that are used.
JMI has given you very good advice on how to locate the place where the check is occuring. If there is a message box that pops up and tells you that the file is corrupt or has been tampered with, then set a breakpoint on MessageBoxA. The crc check is probably near the place where the message box is displayed.

jsteed