It's not quite so simple as total length of the bow being the only determining factor of whether or not sinew is beneficial. Length of working limbs in comparison to length of draw is a better consideration to decide if sinew is the way to go. If the 66" bow has a 12" stiff handle section and the bowyer is looking for 28" of draw or better, then sinew could very well be an asset. Add in substantial reflex and the sinew backing becomes even more of a viable option. Too long for sinew is a relative statement. Other factors must be contemplated to determine whether the sinew will help or just be extra mass. Josh