Often this is very difficult since the system is not even designed when the customer is asking for the answer to this question. Which is actutally impossible.
As a very rough rule of thumb we use 100 requests per second per server. The actual number will vary depending on the application and how the users use the system, but we have found it a good first estimate.
The disk usage for a document system is just number of documents times average size. Bandwidth is number of requests times average size of requests.
You just document all of your assumptions and say that the hardware requirements are based on those assumptions.
