Walt, good to see you are planning to build a crossbow.
My way of thinking is that you find your critical dimensions first, and the rest will follow. For me, that is brace length, then draw length based on the prod you select, and then you have the length of the tiller from the prod to the rolling nut or other system (clap lock, etc.). For instance, a prod with a 3.5" brace length, and an 8" draw length would have 11.5" from prod to total draw length. Width when using a rolling nut I determine by the width of the nut, say 1.25" and you add extra material to each side, say 2" total width or whatever you like. Total length for a crossbow you would span using a stirrup should be long enough or short enough so when you lean down to span it, you can reach the string at brace, and not have the tiller punching into your gut
I myself like lots of extra wood, so I like using a tiller blank about 4" x 3" by, say, 38" or so long. You can always take away, but you can never put back. 40" is even better. Factor in the prod socket depth too, so you have enough wood on the nose of the tiller.