The Planning Tool is great for helping you with an initial design for OCS R2. The wizard will guide you through the process including discovery of design parameters such as web conferencing, enterprise voice, VoIP gateway, Mediation, etc.
It uses this to draw a topology for you such as the one below. This one was for 10,000 users with Group Chat, Web Conferencing, Enterprise Voice, External users and Federation, Archiving and Monitoring. When hovering over each server it gives you the recommended hardware specs needed for deployment. The tools also gives you links to planning guides as well as useful information such as our server user model to understand the params we use to profile user base for the modeling. http://technet.microsoft.com/en-us/library/dd425159(office.13).aspx
You can download the tool here.