An Azure (ARM) VM can conveniently be resized, up or down, while it is running, while stopped or while stopped(deallocated). To resize the OS or Data disk, however, the VM must be be stopped(deallocated).

In the following example, a VM was first resized up to a D2_V2 Standard machine from a Basic A2 machine, while it was Stopped (deallocated)

Then, it was resized back down to a Basic A2 machine. NOTE: This could have been accomplished while the machine was running also.

VM > Overview > Size > Choose a Size > Select