Adult tennis racket head size midsize racket head size 626 cm head size inches 97 sq in stringing 18 x 17 levels professional advanced type of racket all round racket playing properties spin balance 325 mm recommended height from 155 cm weight unstrung 310 g racket balance head light grip size l1 l4 shape oval length 686 cm length inches 27 technology wilson spin effect technology season 2016 player grigor dimitrov special feat