I agree with the Gopher ... I dont think it matters too much. I usually just make a straight taper unless I'm getting fancy. If you start adding reflex or recurve or both, a stright profile for half the limb or something really helps with stability, but a straight limberd bow doesn't need it.
My recommendation on length is (handle length) + (Draw length*2) < bow length, so for a 28" draw length and an 8" handle, you want an absolute bare minimum of 64" ntn. I'd recommend 66" personally.
I'm not saying it's not possible to go shorter, I'm just saying that it's better not to push your luck until you've got some experience under your belt. I have a hickory stave bow which is 54" ntn with a 5" handle, draws out to 26" and is 1.375" wide. It's heavily heat tempered, recurved, and took 2" or more of set. and it was a major pain.
Also, heat treating hickory has been very effective for me in reducing set. I've onyl done this on staves. Never made a HICKORY board bow.