In the context of smart cities, with important population density, Unmanned Aerial Vehicles (UAV) should be certified for flying. Virtualization techniques allow us to run several applications of different criticalities on the same hardware architecture in a spatially and temporally isolated manner. This makes it possible to adapt the certification cost of an application to its criticality level. We consider the case where each application is composed of a set of sporadic tasks that are executed into a dedicated Virtual Machine (VM). The set of VMs is monitored by a hypervisor that provides them an abstraction of the underlying hardware resources. The use of an hypervisor to isolate the applications hosted into their VMs results in a two-level hierarchical system. The VMs are scheduled at the first level then the tasks of each application are scheduled at the second level. The parameters of the tasks composing the different applications are known a priori and the main contribution of this paper is to provide a sizing method for the VMs. It consists in computing both period and slot duration for each VM such that its assigned tasks are schedulable (satisfy all the deadlines of the tasks).