Charms are 5", right? You'll need to trim down the square part to 4.5" to match the HST's which you will also trim down to 4.5" after sewing on the diagonal. Finished size would be 4" per patch or a 12" block.
If I'm wrong and charms are 6"--your HST's will be trimmed down to 5.5" and you'll need to cut the squares down to 5.5". The finished block size will be 15".