There is no way to enlarge this limit just by configuration. This is a technical limitation imposed by the schema and column definition for audit data. You can technically change it by re-defining the various columns using the same limit within the database table, and then setting an optional property in the alfresco-global.properties that informs Alfresco of a custom limit. But then you will have a schema that no longer complies to the Alfresco "expectation" and you will have issues with schema validation and future upgrade procedures.
If you want to keep the full text and don't care about any significant increase in the size of the database, you can change the data extractors / generators of your audit application to produce String values that do not use the java.lang.String class, e.g. by putting them in a serializable wrapper. Then Alfresco will store the value as a BLOB and there is no limit on those. The downside is that Alfresco will not deduplicate the values on the database which means that the size of the database grows much more quickly and identical values might be stored many thousand times...