If your charms are 5" square, putting two together will make two 4" finished squares. Actually they will be a tad larger, but not much. You can trim them to 4.5" unfinished. Cut your squares that won't be used as HST's to 4.5" too, and everything will fit together.
General rule of thumb - if you want a certain size finished HST, add 7/8" and cut the starting squares that size. However, in practice it's easier to add 1" and trim the HST down when you're done.