- Export the Campaign Activity entity;
- Edit the "customization.xml" file (look for attribute/field "objective" and change length to up 100.000 characters);
- Import the "customization.xml" to your CRM;
- Publish your customization (just to make sure!).
This workaround may be unsupported but according to Jim Wang it's pretty safe. I hope Microsoft reviews this 2000 characters limitation and/or the ability to change it.