Karl,
Things to look out for when buying your first surfboard are, like you say, nothing too short. 6'6" might be ok as long as you're not a big guy. Try to find a board with plenty of volume in it i.e. something nice and thick like 2 1/2 inches or thicker with a nice wide plan shape, may 19 1/2 inches or more wide. Also check the rocker, that's the side profile curve of the board, the flatter the better in your early stages of surfing, this will make it easier to catch waves and help the board float over fat wave sections. Also something to consider are the new epoxy type boards, these are more bouyant size for size than polyurethane (foam core) boards and hence easier to learn on, Resin8 do a great range of boards - see Resin8 section on this site.
Hope this info helps,
Cheers,
Matt.