Size depends on the style of kite, your weight, and local wind conditions. If you're a newbie I stongly recommend a Peter Lynn foil (aka twinskin) because:

Extremely stable - always returns to zenith

Self launches

Less likely to crash

Self inflates - No pump needed

Relatively light bar pressure (ie easy to fly)

Usually easier to relaunch from water
There are a few caveats:

Can take on water making it harder to lauch (but it takes 15mins in the water before this is an issue - you're usually back up in in <5)

"Bow-ties" (requires a simple technique to unbowtie)

Can luff & drop out of the sky (rare)
but the above are all really minor issues especially compared to the significant advanatges.
I bought a VenomII in October 2006 as a 1st kite and havent looked back. Its very forgiving and you can concentrate on learning the basics of the sport.
It is also a top end performer. A 16m PL VenomII was used by a 4 top finalist of the mens opens in the 2007 Australian Wave Nationals (actually I think it was the winner).
Also hire a big floaty bi-directional board for a few months till you get your balance.