The bigger the board the easier it is to get up and going. 1cm wider has more effect that 1cm longer. Bigger boards are good for light winds and cruising, smaller boards are nice to jump (one day-when i get there

) and easier to chuck around.
If you want to go surfboard try epoxy as they are stronger. A glass board could be damaged easy. Epoxy is usually shinier, plasticy looking, and in general have no stringer (but are sometimes painted on, but thats obviously done).
Im not sure about concave and flex etc. But i hear the double concave used is good in the chop.
Cardboard is a brand of board.
Less rocker makes it easier to plane, as the board will be flatter,but harder to turn. and more rocker makes it easier to turn and control, but takes more to get it going.