The more information the better when we look at sizing for a new workload. Here are a list of parameters to get started with.  What are the content profiles?  ie bit rates? What is the expected percentage of playback in the library?  ie bit rate 1 = 15% and bit rate 2= 35%, bit rate 3=60% If you get the answers to these questions you can start to narrow down the node type and the library size.  You need to be able to satisfy the number of connections and of course the library size to run the environment effectively.
What are some of the questions and data points which are required to ensure I can size a properly designed Isilon cluster?
Questions to ask for sizing VOD workloads