Hi Nani
My daughter is 48kgs and she uses nothing bigger than a 8m
Its a delta style so it has a god bottom end, is very stable with great depower
On lighter days she uses my 135 board and normal time she has her 128 board
Most brands have kites in the delta shape and if you get 2010 or 2011 the depower is amazing.
The 8m is good for 13knot to 20+ (ellie comes in if it gets over 20 knots)

Enjoy