What performance your organization wishes to achieve will decide the manpower requirement. To decide on the performance, you need to measure ratios associated with each department. This measurement will help you understand what needs to be increased or decreased. Once you determine this, you will know how much manpower you require. While making decisions, it is not just a question of numbers, but the type of manpower also matters.
Finally, I can say that "rightsizing" depends on who can deliver what level of performance.
Thanks,
Dinesh Divekar