I have found that I like my box pleats to be between 2" and 3.5" wide at the hip. I can go more than 3.5" if the man I'm making the kilt for is a larger man. Otherwise, it just wouldn't look proportionate. But anything less than 2" and the pleat tends not to hold as well (since the pleat depth is going to be roughly 1/2 of the width of the pleat).

A "true" box pleat will have little or no overlap between the pleats. A pefect pleat, therefore, assuming you are pleating to stripe, will be 1/3 the size of the total sett repeat.

(BTW, if I know the cloth I'm using won't hold a sharp pleat that well, like the Fraser & Kirkbright cloth the X Marks tartan is made from, or a Harris Tweed, I'll make the pleats on the larger size to allow for a better drape).