A method for testing a mobile telephony network having a plurality of cells whose sizing depends on at least one selection or reselection parameter comprises a step for the recording, on an onboard computer connected to a mobile telephone, of predefined values of selection and reselection parameters for each cell to be tested, a step for the presetting, for each cell to be tested, of the predefined values of the selection and reselection parameters, and for the capture of data obtained by the mobile telephone for each cell, and a step for the processing of the captured data and the determining of the sizing of each cell. The system for implementing this method is also disclosed.