The following are several ideas that have been created to dress up windows where casing is absent. This first example shows a valance that is approximately 10 inches tall with a fancy crown moulding and a bead moulding at the bottom edge that has two coves on each side. The top seems to be rather thin, maybe about 1/2 inch thick and has been rounded some on the edge. This is from hgtv.com.
The next example shows a standard crown moulding, a top that is a bit thicker, and a base cap moulding at the bottom edge. This also looks to be between 8-10 inches tall. The cornice is perfectly sized only extending just slightly more that the drapery as the first example. This one seems to carry a stronger distinction, and a bit more traditional, it is from Tedswoodworking.com.
The following example claims to be 9 inches tall but looks to be 12 inches upon sight. The moulding at the bottom edge is uncertain, and instead of crown moulding at the top, a chair rail type is used with a top extension. The extension from the wall also seems to be a bit excessive which may be to allow draperies that are not shown. This is from 3blindmiceusa.com.
Here is an example of a valance without a top cover. One issue with dressing up a window is to take advantage of the idea of keeping dust off of the drapery. Why not add a top to protect any kind of window covering utilized? The bottom edge profile has been incorporated into the face of the valance providing a nice detail. This is also more difficult to mount to the wall, from alfa.com.
This last example shows a window that has been cased and the drapery hides most of the window trim except the stool. This allows you to have a bit of perspective in how size and proportion affect the overall look. This valance also does not seem to show a top cover and may be a bit tight on the drapery. This is from WindowTreatmentDesigns.com.