Expanding the Relationship Notes field
Modifying field types in CiviCRM is bound to create problems later on (e.g. with upgrades), so I wouldn't do that.

The straightforward approach would be to create a custom fieldset for relationships and add in a new custom field that is large enough for your data. I suspect there might be a few gotchas in terms of accessing this field, but if you're in Drupal, then the entity support in Drupal might solve this.

Apropos of this, here's an article about Drupal that points to how important this kind of fieldable relationship entity can be:
http://atendesigngroup.com/blog/drupals-weak-link