Have you tried the Parallel Dimension tool? Quadrant snap (Q) to the ends of the ellipse. Maybe use the "Move Text" if the dimension is within the ellipse.
If you click on the "Copy" icon in the Selection Info palette, all the Property information is sent to your web browser for easy copying.
You can get a circumference Dimension on a Circle, but not a Ellipse, with the "Angular" dimension tool set to use the "Arc Length" option.
Designer, Deluxe, Expert, Basic, Platinum
RedSDK enabled, except in 2018
Windows 10, 64-bit