Howdy Swedis,
You didn't describe the grain, just any board won't make a bow - it's all about the grain.
If it's perfect grain, you could make three long bows with 6-1/2" wide board.  That's a stiff handled ALB, 1-3/4" wide each.  As far as thicknesses, you want the handle(s) to be about 1-1/4" square, (then you can round / shape the handles). 
I've read that a Holgaared bow is very challenging to make (to tiller), and that it's not forgiving.  
So, if you are just starting out, a Holgaared is like jumping into the deep end of the pool to learn to swim.  You'll learn a lot by trying to make (and tiller) a bend-through-handle bow or an ALB - be sure to back your bow(s) (silk or linen or...).  
Follow some of the Build-Alongs / guides on this site, and you'll be well on your way.
And Enjoy!!
all the best,
Canoe