Junior tennis racket head size midplus racket head size 645 cm head size inches 100 sq in stringing 16 x 19 playing properties comfort power balance 320 mm recommended height 140 155 cm weight unstrung 230 g racket balance head light grip size l0 l1 frame material graphite shape oval frame profile min 24 mm frame max 24 mm length 635 cm length inches 25 technology head graphene touch season 2016 player novak djokovic s