measured by the BB shell, that is how wide the frame is where it holds the bb, usually 68 or 73mm, some frames are less and use spacers, (like my cannondale) some 73mm bb come with a spacer to use with a 68mm shell, there also exists a 71mm bb shell, but that was on old french road bikes, so shouldn't be a problem if you posting in the mtn bike forum, those where also threaded differently

what frame is it, maybe someone here knows the size