Junior tennis racket head size midplus racket head size 645 cm head size inches 100 sq in stringing 16 x 19 balance 330 mm recommended height 140 155 cm weight strung 250 g racket balance even stiffness 68 ra grip size l0 l1 frame materials tungsten carbon graphite shape oval length 66 cm length inches 26 technologies babolat cortex expert babolat woofer system season 2015 series babolat wimbledon competition wimbledon addit