I weigh 54 Kg and am 174cm tall. I started windsurfing exactly one year ago and now my level is quite good.
Here is my tip to you: learn how to water start on your board, then consider buying a new board. Why do I say this? Because once you can water start, the range of boards you can use increases massively. If you buy a new board now, learn how to water start on it and realize you should've taken a smaller board because you can now water start, you'll have bought two boards instead of potentially one.
Water starting isn't difficult to learn in a regular wind. It took me two sessions to learn.
So, now that my speech about improving skills before equipment is over,
I weigh 54 Kg and am 174cm tall. I started windsurfing exactly one year ago. I currently use a 106l board.
Size wise for you, i would go for a 133l Go or Futura. It'll be way enough volume to float, but also small enough to allow you to enjoy more performance.
Depends also on the wind you're sailing in...
But like i said, try to learn water starts before buying a new board, and if you sail in conditions not strong enough for water starting, forget everything i said earlier and dont hesitate on a 133l board,
Hope this helps,