In PCBNew, click on "Design Rules". Then select the "Global Design Rules" tab, and enter parameter values under "Custom Via Sizes".
KiCAD will not accept a custom via size that is smaller than the global "Minimum Via Size". Also remember that your board fabricator will have limits on the smallest drilled hole, and minimum annular ring on the via pad.
After creating a custom via size you may select it using methods similar to selecting a track width.