Long Integer fields take numbers/digits, not chars. Long integers, according to the help files (I just pressed F1 in the field size part for a field) can take numbers up to:
2 147 483 648
The "length limit" for long integer is 10 digits, providing the value is below the limit from the help files that I referred above. You wanted 12.
Simply choose another field size/datatype. If you are going to perform math on the field values, consider another numeric (Decimal?), if not, use text datatype with size 12.
Roy-Vidar