The actual threaded steerer that sticks up above the headset is 1" OD, matching the 1" ID of the riser. Remember it's a threaded setup.

(I am doing this all from memory; I replaced the stock stainless riser with a piece of aluminium tubing on which I clamp the stem. So because the stem is a direct clamp, the riser must be 1-1/8 OD. Therefore the ID must be 1" or a smidgen larger. I know the threaded headset is the larger variety, which comes in 1" and 7/8" sizes.)