You could work out the size you needed on graph paper using the widest dimension you'd need to go across the back, then calculate how large you needed to make the blocks from that. You'd probably want to make the design flow across the two sections of the front, so you'd need to plan for making some vertical strips extra wide if you want to allow for overlap for buttons and buttonholes, plus a facing. The front, of course, would be wider than the back. You could build some darting into the seams. I'd personally have the darting coming from the shoulder rather than from the underarm. I just think this kind of design works better that way. Darting at the waist would be optional. Then I'd try the design out in muslin before doing it in good patchwork. I'd just make vertical strips in the muslin in the areas I planned to make the darts. No need to put patchwork in the muslin for any sections you aren't even going to dart.