I previously wrote a Hyper-V RAM Calculator spreadsheet. During the process of writing Mastering Hyper-V Deployment, I decided that I needed to put together something that covers more than just RAM. The new Hyper-V Calculator spreadsheet covers RAM, disk, clustering and CPU. The CPU stuff is a little rough so you should use it more as a rule of thumb than anything else.
As usual, no promises are made. Use it if you want and do so at your own risk. You can download it from here.