Backup appliance for windows server

1) Determine your busines requirements re RPO and RTO:

https://www-secure.symantec.com/connect/forums/creating-backup-infrastructure-best-practices

2) Design considerations:

https://www-secure.symantec.com/connect/forums/design-consideration-please-share-your-thoughts

https://www-secure.symantec.com/connect/forums/sql-cluster-backup-solution

https://www-secure.symantec.com/connect/forums/netbackup-75-v-ray-technology-sql-vms-and-transaction-logs

3) This is a "must" read:  The Backup Planning and Performance Guide:

https://support.symantec.com/en_US/article.doc7449.html

HTH.

You should ask your Symantec Partner / SE to perform the calculation for you. You'll need to give a break down of the types of data (File System, SQL, Exchange, etc). You should indicate the retention level you require  on the appliance (1 month, 2 months, etc). Based on this they'll tell you the size.

If your requirement is pretty much standard (with little future growth) a 4TB Appliance should be OK. But you should not go on my unofficial recommendation.