what an appliance can handle always depends on the features that you use and on the workload that goes through the box. There is no general recommendation and sizing is a difficult topic.
If you do URL filtering only, a WG 5500 should be easily able to handle the traffic from a 100 MB/s connection. With AV filtering, web cache, authentication and other options turned on this might be different.
Sizing based on the bandwidth is very imprecise and should only be used if you have no other data. Finding out how many requests per second go through the proxy (peak value) would allow a better measurement. Your sales contact will be able to help you with this.