You can change the min and max length fields in the template for the screen. Ex: templates/default/billers/edit.tpl or create.tpl. However, this should be done as an extension so that it isn’t lost when you update. Extension info can be found in the Extension section on the left.
However, the intent of this size if to support the ISO 3166-1 Alpha 3 code for countries. Which might be added as a validation in the future.