Managed to find the attached size chart on web, note your 70kg's converts to approx 154lbs. While recommended sizing given is quite broad (134-142cm), think what you can take away from this is don't go smaller than 134cm. (Keep in mind this is a wakeboard chart, but has some relevance to using boots.)

If of any help, I changed to boots recently and stepped up the size of my board considerably from what I had used for pads/straps. My new board is 140cm and size chart for my weight recommended 138-146cm. Has been a good move.