Usually it´s the external diameter of the fork steerer which is stated when looking for a stem. A "1 inch stem" has 22.2mm quill, "1 1/8 inch stem" has 25.4mm quill.
As far as I know Bullmoose stem/bar combos were only available in 1" BITD, so 22.2mm quill (or sometimes 21.1mm/ old BMX...