What board and sail are you using? What wind strength?
At 54kgs, I'm in the same boat. I use a 29cm fin on my 106l board (one for which you would expect a fin more around the size of 33) and now with the footstraps fully back I never get excessive lift, only occasional spinout in chop. Using mainly a 6.0m and 5.3m this works fine for me.
I've also sailed on a 130l Bic with a 36cm fin with 6.0m well powered up and this was unmanageable. Even with both feet in the straps and good form the board would simply lift up too much and become extremely unstable...
In the past I've tried one different fin on my 106l in light winds, a 34cm Tabou freeride fin I believe, and it felt like I was dragging a plastic bag around. I much prefer sacrificing some upwind angle for additional control at speed and less drag with a smaller fin.
Hope this helps.
JP Super-X 106l, North F8 Ram: 5.8m,