Unless otherwise stated, all have a maximum of 255 characters, although the reality of the need is far less.

The DMC number is actually fixed at 12 characters in the current standard, and the others are dynamic in length - depending on the carrier. The longest parcel code/carrier consignment code is actually 28 characters in the current standards. In other words, if space is at a premium, you can get away with 100 or even 50 characters. And the characters used in carrier reference codes are ISO-8859-1, limited to A to Z, 0 to 9, dot, dash, slash and underscore - they are also case insensitive.