It's not crucial to not have pleats less than 2" wide, but I think in a box pleat, they really do look better when they're wider.

I have a Black Watch from heavy weight material (either 18oz or 21oz, don't recall). I box pleated it, and the pleats are 2.25" or so at the hips, 2" at the waist, pleated to the sett. It looks pretty good pleated that way, and I wouldn't really want pleats much narrower than that (in a box pleated kilt)