Martyn29 - It is indeed 6x4ft in size. The material to use depends on if the board will be portable or not.
For a portable board I recomend 6mm ply with timber bracing. Try constructing having the portable layout in smaller sections that can be bolted together when the layout is in use. This makes storage and portability alot easyer. You will need to cut tracks accordingly using a junior hacksaw or alternativly the Xuron track cutter. However avoid having pointwork and curves accross the join.
For a fixed perminent board, I recomend using MDF or ply 9mm thickness with timber bracings. Remember to wear a mask when cutting MDF
Your local hardware store should cut the boards down to size for you. However you must be careful when you ask them to do this, as some places will just use a normal handsaw to do the cut, rather than a fixed and installed circular saw, so you may end up with wonky cuts. My local Travis Perkins are guilty of this. And I belive Wickes do the same. B&Q use circular saws so you get a straight cut every time. If in doubt I'm sure your local joinery would cut the board for you.
And a final note on wood - you MUST ensure that the wood is not bent or warped in any way. If it is it will cause you no end of problems during construction and trains de-railing (Trust me on this one, I constructed my layout using wood that was slightly warped (It was unfortinatly the straightest the shop had) and I had no end of problems and had to 'pack' up the track in places to get it level. And it still isn't 100%.
I would like to refer you both to this website, it is an exalent site and will help you get started in your project.
http://www.newrailwaymodellers.co.uk/
Also
http://www.themodelrailwayforum.com/index.php
Good luck with your projects!
Hope I've been of assistance
Heinz