Microsoft publishes lots of information about sizing and even has a neat free capacity calculator you can download (Did I mention it was free!). But knowing where to start and what to look out for is harder especially for those who don’t do a lot of sizing work.
In browsing around, I found an interesting post that looks at a proposed sizing for 1500 users, consisting if three Front End servers, plus more. The author was asking for advice and the rest of the post is just that advice.
If you are planning on deploying Lync, this might make good reading and is certainly good food for thought.