We propose a broadband measurement method for measuring the electromagnetic properties of metamaterials in the frequency range (1-6 GHz). In this experimental technique, an original adjustable height strip transmission-line was developed, which allows characterizing several layers of metamaterial. Therefore, the influence on the measured constitutive parameters (complex permittivity and permeability) of the number of unit cells in the sample to be characterized has been studied. This study enables us to define the criterion of the representative elementary number of cells. This criterion must be satisfied in practice in order to determine a microwave behavior for the sample under test very close to that of the infinite medium. Experimental and simulated results are presented and discussed.