The width of the slitlet is not a user setable parameter. The width is specified in the final steps of the mask fabrication process, so you will need to tell the KPNO support person making your masks the desired width. The width is setable in steps of 0.38 arcseconds (the film recorder pixel size corresponds to 0.38 arcseconds). Most observers choose the default 2.5 arcseconds which rounds to seven pixels on the final mask. The minimum recommended width is on the order of two arcseconds.