Hi, and welcome to the site!

A squillion kites for sale in the Buy & Sell (see link above)...

Getting lessons are a great start as you get to try out the different sizes via a school, before buying your own.

But .. it all depends on the wind ... even if you pick the "right" size kite, there will be days when it's not enough, and days when it's too much.

Check out all the starting kitesurfing articles here (includes articles on what to look for when buying a used kite)

www.seabreeze.com.au/Articles/All/Summary.aspx

Enjoy!

Kiting is like surfing in that you wouldn't go off to live at the beach and expect one surfboard to do it all.
2kites is what you want to start with. An 8 and a 12 of most brands would be good. Unless BWS in which case a 6 and a 10! for reasons you'll probably find out later.

If one kite only, I would get an 11 or 12 as it will get you experiencing board work when the wind is not too intimidating and get a big TT - min 145, go bigger if you can find one. The wider and squarer and longer and flatter the better. You'll double the days you can kite, you'll always have days for this board forever, and you'll be kiting upwind way earlier meaning less walking up the beach and more time kitesurfing.

Talk to a shop and work a deal on new or used or demo models with lessons thrown in. In most cases you'll want at least 3 lessons.
Happy sailing