No they do not have to be the same size ... just kinda close for aesthetics.
The easiest way I know of to make scallops is to make a long freezer paper band of paper about a bit wider than the scallops you intend to make. ex: 2" deep scallops use a 3.5"-4" width of freezer paper. Then fold that band continually til you get the size you want. You can practice with folding in half then half again ... or
thirds then thirds again or half again. The object is to keep folding til you get the size you want. Do this for both the width and the length of your quilt. This way your size is tailor made just for that quilt. If this is a size you regularly make, keep notes.
You can then take a saucer, bowl, dish, or whatever to make the scallop and cut the folded freezer paper. Guess what, you can then iron the pattern onto your quilt as a guide for cutting.
I prefer this method to buying a scallop ruler and then trying to make it fit ... which you can do if you prefer.