There are four key resources you should focus on—processor, memory, disk, and network, as those resources define the core resource components of every virtualization project. It is important to gather those metrics from the physical environment and you should capture such data for a longer time—a minimum of a week, but preferably even longer  to discover any specific usage patterns over the time or peaks.

As a best-practice you should capture the performance data during a representative time period such as during business hours or during the running of business-critical applications (if they are not run constantly).

Only if you have the representative data, will you be able to correctly right-size you environment and cover any peaks and spikes in the resource usage.