Capacity planning falls into two general categories. Planning for existing systems and planning for systems under construction. For existing systems, you can gather metrics over time to see how the system is currently growing and then project that out in a line to see when you may need more disk storage, additional CPU capacity, etc. For systems under development, you have to get estimates on how much data will be stored, how big that data is, how many users will be accessing the system, what the anticipated transaction rate will be, etc. There's no simple, straight-forward, if you have A, you need a server sized X. It really depends on the behavior and growth of the system you're managing. Best thing to do is start gathering metrics now.