Make it longer at least 70" IMO. 72" is a reasonable start... you can always cut it shorter... especially as you near final tiller and put on decent nocks. The clue is in the name
66" is ok if you are 5'4" tall and drawing 24-26"...
Keep the tips a bit wider with temporary wooden overlays glued on the back. This allows for sideways adjustment to stop twist and keep the string line right. They can try and go sideways on you especially if there is some reflex.
Once it's back to about 20" you can start narrowing the tips.
I normally mark 'em up at about 30mm square over the centre section then the ends tapering to about 20mm square. The final stages of tillering slim the tips right down.
If you are not sure of the quality of the wood go for a slightly wide flatter belly more like this 'D' rather than a steeply arched one.
Del