Adult tennis racket head size midsize racket head size 626 cm head size inches 97 sq in stringing 16 x 20 level professional type of racket tournament racket balance 310 mm recommended height from 155 cm weight unstrung 310 g racket balance head light grip size l2 l4 frame material graphite shape oval frame profile min 20 mm frame max 20 mm length 685 cm length inches 27 technologies yonex isometric yonex 3d vector shaft yon