Hi teksup88,
You're correct - when you're sizing your Exchange virtual machines, you should focus on the CPU and memory sizing requirements for Exchange itself. Those requirements include the requirements for Windows, such as allocating a proper page file size (for example). On the storage side, you'll probably need to allocate at least 40-50GB for the system drive to allow enough space for the Windows binaries themselves and possibly the page file (unless you redirect it to another drive). Beyond that, make sure you follow the sizing guidance for your particular Exchange role.
The other thing to consider is that the sizing guidance from Microsoft assumes that no other software is installed on the server. If you've got anti-virus software, backup agents, etc., you'll need to work with those vendors to understand their requirements so you can factor that into your CPU, memory, and disk sizing.
Does this help? In general if the server is only running Exchange then I would focus on the proper sizing that Microsoft provides for each Exchange role and go from there.
Matt