Any sized material can be used, the important thing is to space the center hole exactly at the mid point of the two outside dowel holes in this manner:

Drill a pilot hole for the wood screw in the center of the bar. Place a common nail through the hole and tack bar to a piece of scrap material, do not drive the nail all the way in. Clamp scrap material to drill press table and drill hole for dowel in one end. Rotate bar and drill second hole.

Use whatever size dowels you have, the ones shown are 1/4".

Insert a wood screw to just protrude through the bar, you may have to touch up the point of the screw with a file.

The dimensions shown are a good size when working with 3/4" and 1 1/2" thick material. If you are working with larger material just make a gauge with the dowels farther apart than the thickness of the material, the dimensions are not critical. The screw can be replaced with a pencil if the bar is made from thicker material.