If you find your beret too large, cut a small notch in the rear, add a wide drawstring to the hatband, and pull it to size, then tie in the back. This is way all Scots Bonnets, and other headwear, were sized, before the era of machine made hats, and elastic. When you knot the back, sew the bow knot in place, so it will not untie, and trim the ends to your preference.
Mr.C.Anthony
Historical Tailor74th Regiment, Argyle Highalnders-Re-created-.