As with many aspects of interior decorating, there is no "correct" or "right" length and width for valances. Whether you're trying to choose a valance from the store or make your own, you have a surprising amount of wiggle room when it comes to valance measurements. Although there aren't exact rules to follow, there are some guidelines to bear in mind when choosing or making valances.
The horizontal width of a valance doesn't necessarily have to be an exact figure. While the temptation is to simply measure the width of your window and use that measurement for the valance, this is actually incorrect. Remember that, just as with the curtains, the valance is going to be scrunched up on the curtain rod. When measuring fabric for the width of the valance, multiply the width of the window by 1.5 to 3. Use a number on the low end of that spectrum for stiff fabrics, such as canvas, that won't scrunch very much.
You have quite a bit of freedom with the vertical length of your valance, too. There is not an exact ratio to use for valance length, but a good start is around one-sixth to one-quarter the height of the window. However, if your windows are very wide, you can go a bit longer than that. The best thing you can do is pin some scraps of fabric so they're at different lengths, then hang the pieces of fabric from your window to see which you like best.
Making the Valance
Once you have your fabric measured and cut — don't forget to account for hems and the loop for the rod — you need to pin the fabric. Fold the hems and pin them. Stitch all of the hems with a sewing machine, removing the pins as you work. Once the hems are stitched, you need to make the loop for the curtain rod. To do this, you can just wrap the top of the valance around the curtain rod and place pins to mark the measurements. Remove the rod, add more pins, then finish stitching.
Scarf valances are an entirely different entity because they don't have to be exact at all. You don't even really have to stitch them if the edges are clean enough to not need hems. Scarf valances are simply draped over the curtain rod the way you might wear a scarf, with the two ends just over your shoulders to give the impression of wearing a scoop-necked shawl. The ends hang low, to at least the midpoint of the window, and the center hangs scrunched over the curtains.