To measure your back, take the point running between your two hip bone tips, what you can press onto your side, then from the middle of that line, go up to the big vertebrae in your neck, that sticks out more than the others, and which is about shoulder height. That's your torso measurement.
width I never heard about, the only area there I can see mattering is the width, or separation, of the shoulder straps where they attach to the pack, which is based on what it is roughly beteen center of shoulder to center of shoulder, flaring out. For my pack, that was about 3.5 inches, give or take, at where the straps connected to the pack.
Pack volume is trivial to calculate, especially if you are the person making the dimensions, if the sides are basically rectangles, the old: area of base times height. So width times depth times height, in inches. Divide total by about 61 to get the liters size.
I ignored the rolltop volume when calculating my pack. Be careful, there are some, I assume older, designs out there that are huge. For shoulder strap only I wouldn't want more than 30 liters, if that. I also ignore external pocket volume completely, I don't consider that part of the pack size, 4 bottles and some stuff stuffed in the back tall pocket.
Straps are hard. There are J straps, which curve towards the bottom, going straight out from their attachment points, and there are S straps. S straps require a reasonably stiff foam to work, otherwise the S collapses, as I discovered during testing. Evazote foams, while delightfully soft, are not stiff.
Do not make a real pack first, you will spare yourself great pain and sadness by making at least one prototype, with the volume and straps you want. If you make it reasonably well, you'll have a simple bag backpack to use for something even if it's not perfect. Since you are starting from zero on straps, I'd make a raw prototype that you can rip the straps off easily to test their fits until you get a fit you like.
S Straps take more work and trial and error, but fit to your body better, J straps are simpler, I went with S straps, but it took m e about 6 or 7 prototypes before I got close, and what I ended up with was not perfect, but it was fine for long backpacking trip, at least so far, so I guess it was ok.
Straps are this: length that is sewn onto pack. Length from pack body to center of shoulder. Length from center top of shoulder to well below armpit, if J strap. If S strap, it's harder, the curve starts a bit below the front of the shoulder, inward, then back out to get under the armpit, then curves a bit more out, it's not really an S, it's more like a sort of lightning bolt, simplistically put.
Those dimensions are your body, so you have to figure it out on your body. If you have a friend or significant other to help you, it's hugely useful, since they can measure these distances for you, it's very hard to do it yourself.
Straps there are different places, you can attach them to the seam where the top cover or rolltop hits the pack body, you can attach them a few inches down, just remember, with no real hip strap to hold the pack stiff and firm, how it actually acts with weight in it might not be what you expect, which is why you make a prototype first, then take an hour or two hike with it, to see if the straps, size, fit, etc, actually are what you thought.
I've read more than one backpack maker who said it takes 3 to get it right, that's my experience, my first attempt I ripped up since it was all wrong, way too big, and everything else was off too. I got a working version in the 2nd attempt, after taking it half apart to fix straps that were too tight.