Design Line doesn't seem to be designed w/Oval in mind. After your design line is drawn, go and edit the design lines. In that dialog, you can add an Oval constraint.

The other thing is that Oval sizes aren't true sizes, you may need to either input the true stretch out sizes or make sure that the Effective Diameters are setup properly if you want to use nominal sizing.

Can't say as I'm real familiar w/Oval myself but know enough that I don't want to draw it w/Design Line.