Which model do you have now?
What features do you have turned on?
SSL Scanning? (What % of SSL?)
As a first guess, with everything turned on and 15% SSL, i'd estimate 2 WG5000's or 1 WG5500.
MWG 7 Sizing Calculator 2.0, 16. September 2013, 05:26
System: MWG WBG-5000-B Traffic: 400 requests per second with with 15% HTTPS.
1 system(s) (91% utilization)
2 system(s) (46% utilization)
We current have an an IronPort WSA s370.
We are currentlu using McAfee VSE 8.8 managed by McAfee epo 4.6
We do not current have any AntiVirus on the Proxy/Gateway as it is managed by the upstream proxy
We do not do any scanning on SSL traffic only HTTP traffic.