don't know what kind of wind port douglas has (hot gutless wind perhaps?) im 75 kilos and go upwind in 17 knots with the 7 meter in waves on a t/t and 16 knots on my shiny new underground freewave



... i would go the 7 meter if your looking at 20 to 25 knots @ 70 kg's if your into boosting.
even a 6 meter for your weight and wind range could be ok if you like the waves more than boosting and/or dangle tricks.
personally i think north fudges kite sizes a little... as this year laying a 07 north rhino 9 meter along side my 09 7 meter rebel they looked identical in shapes and sizes...