I am guessing this has become some sort of a standard exercise for technology students - a similar question was asked of me by a student in Singapore.
Bandwidth requirements are determined first by business objectives, then business requirements, then technical objectives. The technical objectives determine the applications, the applications define the bandwidth needs per user, and then the user count and location drives the specific bandwidths.
The WAN design will also be impacted by security issues. The public service video streaming and Web site may be better served with a hosted solution seperate from the company Intranet, for example. Access to the hosted site might be limited to a very few employees or platforms that need to interact with the hosted platform for content control or other support. Internet access might not be a requirement at all for the company on a per user basis, or may be needed for every employee.
Availability is another issue. The cost of bandwidth varies widely geographically. Optimizing bandwidth usage is not always cost effective. Like storage and memory on workstations, costs can come down to where efficiency of code means less than time to market.
