The invention relates to a device (1) and method for selecting the size and shape of a prosthesis and/or for determining the location of marks to be made on the external face of the skin of a patient intended to receive such a prosthesis, for example for repairing a hernia, comprising a sheet (2) of transparent material, the said sheet comprising: —a plurality of graphical representations (3a, 3b, 4a, 4b, 5a, 5b, 5c, 5d) concentrically embodying the respective outlines of various shapes and sizes of prosthesis, —a plurality of first holes (6a, 6b, 6c, 6d, 6e, 7a, 7b, 7c, 7d, 7e) arranged along the transverse line passing through the center of the said sheet, and—a plurality of second holes (8a, 8b, 8c, 8d, 8e, 8f, 8g, 9a, 9b, 9c, 9d, 9e, 9f, 9g) arranged along the longitudinal line passing through the center (C) of the said sheet.