Adult tennis racket head size midsize racket head size 645 cm head size inches 100 sq in stringing 16 x 19 string tension 23 27 kg playing properties power balance 320 mm recommended height from 155 cm weight unstrung 295 g racket balance head light stiffness 72 ra grip size l0 l5 frame material graphite shape oval frame profile min 23 mm frame max 26 mm length 685 cm length inches 27 season 2017 series head challenge li