I would do what Jawge said. I haven't made many longer narrow BITH bows, but with shorter ones I follow the grain and rough out about like everybody said, 1.25" wide and 1" thick or whatever, and I basically make the whole bow those dimensions to start. Then I narrow or taper the limbs side to side however I plan to.
At this stage it is obvious that the middle is the most likely spot to bend, right? I spread my arms wide and grasp the bow as far out along each limb as I can and pull the bow against my knee. When the middle just BARELY bends when I pull pretty hard, I mark out the middle and leave it alone from then on, and only work outside of those marks, thinning the wood in the next section of the limb. Then the next. BY then I have it on the tree, and it's often a question of just taking the weight down systematically as you work it to full draw.