Martyn4950 said..
When you say North, how far do you mean? Its quite a big country over there .....isnt it ? lol
If you've got wheels 400km/4hrs north of Perth will get you to Dongara, Geraldton and Coronations easy enough. Leave after breakfast and be on the water after lunch time. Stay for a couple of days then head back when the body can't take another session.
Guys up here do use 5 and 5.5 on surf boards but a 5 on a twinnie isn't too common. I'm on twinnies and my smallest kite is a 6 Ozone Edge - that'll take me up to the 35-40+ range and I'm slightly lighter than you.
If you're staying in the Perth region or South West I'd say the 7 and 9, but if you could leave other stuff out then the 12 will be handy. Pack light, you only need 1 pair of board shorts and a couple of t-shirts. Scrounge an old pump from someone on here when you arrive.