In this work, useful criteria for dimensioning X-wave launchers are provided based on the definition of an original metric of confinement. The mathematical expressions for evaluating the nondiffractive features of a spectrally-flat X-wave are first reviewed. Then, a novel criterion for dimensioning the aperture size of an X-wave launcher is proposed to obtain a pulse which is confined over both the transverse and the propagating axes. Such an analysis may be particularly useful for the development of X-wave launchers especially in the microwave/millimeter-wave range, where the available fractional bandwidths are subjected to severe technological constraints.