first of all, in the McAfee Partner Portal you can find the actual sizing guide. This helps, if the requests/sec are not known by the customer.
Perhaps this two documents are helpful for you.
4500 - C. If customer wants redundant PSUs: 5000.
Include redundancy, at least N+1.
For more than a gut feeling, can you give us some indication of volume for HTTP and HTTPS (not the size of your internet connection), requests per second, etc.?
Or you can check the partner portal (if you are a partner), ask your reseller (who will have access to the sizing info), or McAfee SEs.
Well yes we are partner for mcafee and is that such tool available on partner portal? Would you please guide me or provide me the location?
I can't give you the precise location in the portal (due to no missing access), but assume that there is a search. Search for: "mcafee web gateway sizing". That should result in "Sizing Guide for forward proxy scenarios".