depends on how many days you want to kite for.
i.e. if you want to kite everyday there is wind range is approx 8knot-37knots then you won't be able to do it on one kite.
If you don't mind missing some days the a 9m or 10m bow/sle will do and you could always buy more boards.
OR:
Buy one kite for the surf/high winds and one kite for flat water/lighter winds.
********* MASSIVE PIMPING FOLLOWS ***********
I have two bow/sle kites for sale, an 8.5m and a 12m, both of which would do for most conditions, both of course would be perfect.
********* MASSIVE PIMPING ENDS ***********
If for some reason you don't buy mine

then look for similar quality/year of kites in a size that suits you. Lessons always help as well.