I'm not the yew-go-to-guy for sure but I've owned and operated a couple and have some staves.  I cut down a tree this summer and made staves.  I peeled the bark while the staves were green and none of the staves checked.  I kept them in a cool basement over the summer and I wouldn't hesitate to make them into bows at any time.  But with any bow I make, I look for moisture during floor tiller and determine how much more drying time I want before proceeding.   I dry as I go.
As far as dimensions, I owned a 62" flatbow that was 55@27, stiff handle.  It was high quality wood.  yew is light so you can get away with longer bows, especially with an 8" handle.  Personally, I would start out at least 66-68" with the rounded belly.  Width wise you are fine.  For 64" I would go flatbow and 2" wide for starters.