I like 8-10" for ideal, but down to 3 or 4 inches will work as John said. With yew, the search will be for clear sections with straight grain - no spiral twist which shows even in the bark. Yew tends to have lots of small limbs and lots of small knots associated with them. If you find a 6' clear 10" logs it will be very rare. Look for 36-40" prime sections that could be split and spliced at the handle as sister billets. Glenn St Charles' book and video called "Billets to Bows" are excellent aids for working with yew from harvest to finsihed bow.