If you only intend to do some Belly boarding, i.e. just riding white water straight towards the beach from waist depth of water then the correct size is less important than if you are using swim fins (flippers). Personally we believe that it is pointless spending over £100 if all you want to do is Belly boarding.
You'll be amazed at what you can get for £30 to £80. Look for a board that reaches your belly button when stood on the floor, an extra inch or so may give you a bit more flotation. A bit more width in the board will also help.