I would say 1/8" thick, maybe 3/16" and probably 16" long for 64-68" NTN bow. Shorter bow shorter power lam... Do a nice gradual taper on it and you should be good to go. Use hard wood Maple, Hickory..etc. ? One thing I have noticed is using a power lam I'am not touching the belly wood at all. Just doing a width taper, makes life easy I think, but that depends on the design. I usually glue the P.Lam on the back. If I built a forward riser bow I would glue the lam on the belly side.