I think when u are beginning that you are less efficient with the wind therefore you need a slightly bigger kite than when you are proficient ..
I have a couple of friends that are about 75kgs .. they have learnt on 9m rebel. after 2 seasons they still using the 9m and also a 7m. I would suggest 9m for you. but keep your 7m as you will need it when you are better .. and for days of 20knots plus ..
Also depends where you are kiting ... if you mainly in the ocean the wind is always a bit stronger so the 7m will work well ... if you mainly on the river the wind is usually lower then you will need a 9m.
Have fun .