Bessel-beam launchers are promising and established technologies for focusing applications at microwaves. Their use in time-domain leads to the definition of a new class of devices, viz., the X-wave launchers, that are currently under theoretical investigation. In this work, we discuss the focusing features of such devices with a specific interest at millimeter waves. By reviewing the mathematical expressions that describe the spatial resolutions of these systems, we establish here novel operating conditions which might be particularly appealing for specific millimeter-wave applications. In addition, alternative criteria for designing X-wave launchers are derived, showing that an electrically-large aperture is not strictly required, as it seemed from previous works. However, it is shown that the use of an electrically-small aperture would demand a considerably wideband capability. The various discussions as well as the …