Junior tennis racket head size midsize racket head size 613 cm head size inches 95 sq in stringing 16 x 17 balance 250 mm recommended height 104 116 cm weight unstrung 179 g grip circumference 100 102 mm racket balance head light grip size l0 frame material titanium shape oval frame profile min 19 mm frame max 19 mm length 533 cm length inches 21 technology wilson quadriform technology player roger federer series wilson j