im getting a threadless stem soon, but i dont know what clamp size to get.
im most likley going to put the bull horns i put on my threaded stem, or risers that fit in the risers too.
whats the clamp size for them? either 22.2 mm 23.8mm 25.4mm
and i would measure them but i dont know if i measure across the width ,or around the bar clamp

You're doomed. Get a 31.8 stem and buy shims to fit 26.0 , 25.4 , and 22.2 . You'll be able to fit any bar you're likely to encounter.

but im cheap

You could cut a can carefully and use that as a shim. I guess it doesn't have to be a can, but they're easily found, easy to cut, and easy to bend into themselves.

Buy a 26.0 and a 31.8 and you won't have to worry about it for a long time.