I suggest you read the article written by George Winter which is available from this VERITAS link. You will see the article at the bottom of the page titled "VERITAS NetBackup Software Performance Tuning"
http://eval.veritas.com/downloads/van/van_news_nov_i.htm Click on the "Read More" button and it will bring up a pdf document.
It covers many different aspects of an Enterprise NetBackup environment including siizng of servers and storage.