i would say the 12m would be better suited but you are in perth i just read and they do have a lttle stronger winds than us on the east coast so 10.5 could be alright but you would be on the edge of the lite wind side of it but good for the 20knot days.

sorry dont think i helped much

Jacko

Chook,
Are you looking at the 2010 Cult ??? apparently they are better in lighter winds than previous models which will help. I started in Feb this year in Perth and are of similar weight and went the 09 12m Cult............great kite in 20 knots plus, takes a bit of flying in lighter winds, Enjoy!!!

Depends on the year.

I'm 85kg and I had a 12m 2009 Cult and I was happy with it.
2010 apparently improved but still for 90 kg person I would recommend a 12m.

Hey chook,
Firstly, I'm not sponsored or have any alliances with Naish, kiting 10 years, but I am a huge Cult fan - most fun kite I've ever ridden for my style of cruising/free-ride. They're touted as all-terrain so they're very versatile kites.
I'm 83kg 179cm (about 5'10-11") and ride a 12m Cult 2008 (about to upgrade to 2010) here in South Aust.
I'm fairly experienced and enjoy my 12m in 14-30k but don't recommend a beginner riding 12m in the high end of this scale - my point is how versatile they are.
Being your size you might risk the 10.5 being a bit small but I don't live in Perth and hear it gets pretty windy - so it could be ideal.
You could consider a light wind board (even a surfboard) so you can get some action on light days - but be warned, kiting on a surfboard can damage it.
I'd say on a 10.5, as a beginner, with a light wind board as backup, and a normal twintip, you'd be having fun from about 15k to about 24k and that range would expand (in both directions) as you get better.
Ideal beginner kite - will fast track your skill and therefore enjoyment.