You can re-map what Creo parameter populates the Windchill "Name" parameter, but I don't think you're going to be able to limit the length. Creo allows 80 characters for parameters and I believe this is hard coded. I'm not sure what would happen if you limited Windchill to less characters that what Creo allows.
By the way, the mapping between Windchill and Creo is handled in Preference Management under Workgroup Manager Client --> Mapping System Attributes and File Properties. Changing this mapping may require you to modify the download service as well.