5G wireless communication envisions unprecedented changes in current mobile networks in order to guarantee significant higher performance mainly in terms of data rates, latencies and efficiency. This paper provides an analytical model based on stochastic geometry, in order to address the planning and the dimensioning of 5G Cloud RAN. The results shows the requirements in terms of number of virtual base-band units, and the energy gain achieved by virtualisation both in micro-cell and pico-cell scenarios. Thus, the proposed methodology represents a step forward to analyse and compare traditional RAN design with the emerging Cloud RAN paradigm.