It depends. If you want your windows to look taller than they are, Measure about 8 inches from your ceiling and then measure down to where the valance covers at least 8 inches of the tops of the windows. If your windows are fine, measure 8 inches above the window and then go down an additional 12 inches. I do not know if these are hard and fast rules, but they were given to me by a drapery maker. Good luck.

Usually around 15" should be a good length!

It depends on the height of your walls and the size of your windows. Generally you do want to cover the top of the window frame and, if applicable, the headrail of the blinds or shades in the window. Having the correct proportion makes a huge difference in how the finished treatment looks in the room. As a general rule of thumb, 1/4 to 1/5 of the wall space is a good guideline. But variables do apply: for example, for a valance over a kitchen sink with minimal space above the window, I will create a smaller valance (that remains in proportion to the wall and room) so that the valance does not cover too much of the window and reduce too much light & view.