How do I dimension this diameter?

It's not a true circle so you can't add diameter dimension over it

As I mentioned in another post, related to this part, you can use a ref. circle from your model to add  diameter dimenison

Eric Eubanks,

You are obviously a student.  While I applaud your resourcefulness in finding and using this forum for help it appears you are trying to run before you can walk.  Please, do yourself a big favor and invest a few hours in going through the built-in SOLIDWORKS Tutorials under the Help menu.  So much of what you have already asked would be answered, plus you will learn a lot of great techniques.  Your modeling will be faster, easier to edit, and you will more easily be able to achieve your objectives.

Happy hunting!

Perhaps you can use the sketch from the model? Show it in the drawing, and select it there. That may not be the correct answer, but that's what I would do.

If you want the OD instead of the center of the Sweep, here's a way:

1.  In your Part, start a new sketch on the Top Plane.  Place a circle with it's center coincident with the origin, and then you can place a coincident relation between the circle and a point where the shaft joins the spiral sweep.

2.  Go back to the Drawing and show this sketch on the Plan view.  Place a sketch circle in the drawing view that's coradial with the Part sketch circle (using the "Convert Entities" tool is a simple way).

3.  Hide the Part sketch and place your dimension.

4.  If you don't want the sketched circle showing then create a new Layer and turn it off.  Move this circle into it.

It would be nice if you could just dimension the circle from the Part sketch, but if you do that and then hide the sketch then the dimension will disappear.