According to WSP, chg_ref_num has a string length of maximum 30 characters.
For change orders, if the last OC is OC9999 (6 characters), the next will be OC10000 (7 characters).
However, let me know if there is a configuration for indicate that change orders would be have a limit of 6 characters as example.
Release : 17.3
Component : SDM - Configuration
The place where you can create the format for your Change Order numbers is in the Administration tab under Service Desk -> Sequence Numbers. It lets you specify a prefix and/or suffix, but not the number of digits in the sequence number.
The sequence number is not limited to 6 characters, and the next sequence number after OC9999 would be OC10000
Here's where you can read more about this.