Hi
You can create class diagrams as big as it is required. But remember the digram should be clear and easily readable. If your diagram is easily readable it dosent matter how big it is. Also donot leave any thing in the class diagram like method parameter, value objects etc they should be in the class diagram.
Also provide some notes\comments for each and every class\ejb\dao\helper classes etc in the class diagram. by providing this, it is easier for the examiner understand the diagram.
Thanks
Tarang Agrawal
SCJP, SCBCD, SCWCD, OCMJEA