For beginners wider and longer is generally better, however, since you are using red oak and are only looking for 40 to 45 lbs, if you let it bend through the handle, some, you can get away with it being a little shorter. I just built a bendy handle bow for a friend from a white oak board. This is what I did. I started with a 1/2 inch thick piece of white oak 1.25" wide and 60" long. I marked off 8" in the middle for a 4" handle and 2" fades. I DID NOT TOUCH THE THICKNESS IN THIS AREA. It will bend, but you do not want it to bend too much. I put blue painters tape on each end of the "riser section" to remind me. I recommend gluing a backing material for protection. I used linen glued on with Titebond 3, but you can use paper, silk, etc. Start your taper at the end of the handle fade and do a pyramid taper to the 1/2 in tips. Cut your nocks an inch from the tips which gives you A 58" nock to nock bow. This design has a slight thickness taper to the tips, but not much. Mine turned out 30 lbs at 25" and about 34 lbs at 27". If you want it closer to 40lbs, especially since you are using red oak instead of white oak, I would make it 1.5" to 1.75" wide to start. I would keep my handle at least 1.25 wide to avoid too much bend in that area. Then build up the handle with leather or rawhide that will give a little when the bow bends for a more rounded comfortable handle. I am no expert, but have built a few successful bows. I hope this helps some.