A database will usually occupy the bulk of the disk space that you use, and the task of getting the sizing right is far from easy. However, the non-database requirements, although small by comparison with the database, will themselves be fairly hefty, and should not be forgotten during the sizing exercise. You need to make allowance for any disk resilience that will be, built into the system. You will almost certainly be using disk mirroring or RAID 5 extensively throughout the system. You need to calculate the overhead involved and factor it into your capacity plans.