The actual configuration will depend on your business requirements. You should view your Test server as a production-like platform that you can use to test anything before it is pushed onto production. Performance and large amounts of users are also items you may want to test before you run into problems with your production machine.
Usually, your production system will be rather clean when compared to any other platform. Imagine that you have various testing setups, all with their own data and framework. You will need a multiple of the production disk space in your test server to run all and any tests you can imagine.
In addition to that, disk space is rather cheap these days. If you do not want to hit a wall in the near future, include a wide margin in disk space calculations. For example, start with twice the size of your production machine (1TB).