Once you have enabled the serial numbers module, you can start adding or importing products with serial numbers. When adding a new product, check the box next to "This SKU requires Serial Number" and another line will show up asking you to enter the Serial Number Length, which can be up to 50 digits.
Serial number length is not a mandatory field. If it is defined however, the serial number must be of that length.
Serial number length can also change over time.