To the best of my knowledge, the field lengths are not changeable. That field is intended for Ancestral File Numbers only. Spot checking my own genealogy, AFNs seem to be in the form of XXXX-XX where X represents any alphabetic or numeric character. So 12 characters should be long enough to hold any AFN.
Are you trying to use that field for some other purpose? Can you use a different field instead?
Or can you use a tag in the notes? That way there should be no limit to the number of characters used. (See "Using Tags" in the Help menu.)
Tags can be searched for using an Advanced Focus Filter > Notes > Contains > SPECIAL-NUMBER: OK, then "Show selected only" will display a list of only those people who have that special tag.Likewise you can quickly search the notes to find the individual with any particular number.
Someone else on this forum may have other suggestions.