When I started out, every time I made a tenon I had to use my calipers to measure the diameter, then carefully scribe that somehow onto the work piece (which is a mission, if you have to use the tailstock to hold it), and then I could proceed to make the tenon.
Now I know that the really experienced wood turners do this by eye sight. Well, I am not that good and my eyes are getting worse. So I made myself three little tenon sizers:
They are made from 5mm plywood (the good model builders’ stuff). Cut out rectangles big enough so that the outside edge of the rectangle just about covers the jaws at nominal diameter.
Then measure the nominal diameter of your chuck, i.e. when the inside and outside of the jaws form a perfect circle (or look it up on the relevant website), mark a half circle on the plywood, with the centre offset to one side, end extend the lines straight to the edge to get the U-shape. Now cut out the shape with a scroll saw or jigsaw. Use a rasp or file to either just clean up the inside (for gripper jaws) or to file them at an angle (for dovetail jaws, which I use exclusively). Check that they fit snugly over your jaws.
How to use: You can use them to mark the tenon size with a pencil, or you can just eyeball a tenon a little larger than required. Then simply use your parting tool or skew chisel, whatever you use to shape your tenon in the right hand, sizer in the left hand, and reduce the size until the sizer fits. Job done.