In addition to the Nbu VMware admin guide, there is also this document that gives you additional information regarding VM backups:
Support for NetBackup 7.x in virtual environments
http://www.symantec.com/business/support/index?page=content&id=TECH127089
The closest thing to a Nbu sizing guide (They just don't call it that).
Excellent read btw. Most of the things still apply to 7.5.x:
NetBackup 7.0 - 7.1 Backup Planning and Performance Tuning Guide
http://www.symantec.com/business/support/index?page=content&id=DOC4483
The two documents don't seem to address your specific VMware query directly.
Hopefully you'd still find them helpful in narrowing your search.
Though you may know this already, the following points from the vm guide regarding the use of Resource Limit are very important:
Note: The Resource Limit screen applies only to policies that use automatic
selection of virtual machines (Query Builder). If virtual machines are selected
manually on the Browse for Virtual Machines screen, the Resource Limit settings
have no effect.
Note: To limit the number of simultaneous backups per policy, use the Limit jobs
per policy setting on the policy Attributes tab.
I think this whole vStorage backup method resource limit/sizing topic depends heavily on the actual environment; some could afford to handle a more demanding and vigorous backup architecture and some couldn't.
It may sound redundant, but I typically use the "limit jobs per policy" setting to prevent individual policies from stressing out the VM environment (especially if there are many VMs in a policy), and use the Snapshot limit to control - more or less - the same thing on the global scope. (If I'm using the Query Builder).
