You could get away with a 134 cm board or upwards.
There are several things to look out for when buying a wakeboard.
Size too big and you will find it difficult to spin/do tricks, too small and it will sink to easily. Go for a 133-138cm
Tip/tail shape - rounded are good for beginers as they dont catch on the wake.
fins 2/4/6 are all possibilities, some are cupped to allow easy surface spins, i would go for 4 or six as they give you more options.
basically i would advise not getting a so called *beginners* board as you will soon outgrow it, as the learning curve in wakeboarding is quite quick.
Makes to look out for are :-
Liquid Force
Hyperlite
Jobe
Depending where you live i would get down to Milton Keynes tow and test ride some different boards (you can do this at princess too), then you get to try loads of boards and if you buy one you get your money back (for trying thm out).
here are some good links :-
http://www.craigcohoon.co.ukhttp://whitecap_holding.stodge.org/w.../cabletest.htm
http://www.princes-shop.co.uk/
hope this helps