I have a scallop tool (not a ruler, more like an individual template) that is adjustable to make all size scallops. I use it a lot either for making a scallop borders/edges or use as a quilting pattern. I often flip it over for a chain effect when quilting.
Unless your quilt is square you most likely will have different size scallops on the sides than the top/bottom. You want to keep them nearly the same size however otherwise it won't look right. You will also want to consider the corners and round them as well. I put a half of a scallop going one way and the other half going the other way on the corner so factor that in as well when you are planning your spacing.
You will need to bind the quilt with bias binding rather than straight of grain binding. I use the same width as straight binding. You'll need to ease around the curves and massage the binding a bit at the in curve but it should fall into place nicely. Don't worry about the tucks in the in part of the curve. They mellow out.
I mark my scallops with a frixon pen and then stitch along on the markings. I then cut close to the stitching and then put the binding on. This stitching stabilizes the edge. If you cut before you stabilize the edge is likely to stretch. I've also seen where some sew the binding on before cutting.