Junior tennis racket head size midsize racket head size 613 cm head size inches 95 sq in stringing 16 x 17 balance 310 mm recommended height 128 140 cm weight unstrung 225 g grip circumference 100 102 mm racket balance head light grip size l0 frame material titanium shape oval frame profile min 20 mm frame max 20 mm length 635 cm length inches 25 technology wilson quadriform technology player roger federer series wilson j