Putting a new fuel Tank in 23 ft

[justhecs]

took out the old alum. tank that i think was a 100 gallon and in the process of having a new one built. Under the tank was a sheet of marine plywood and that was glassed in and was and wet and water logged. I took all of this out and now i was wondering what to put back. What size should the marine plywood be that i will glass back in place. It seems that you don't have much of a window hear because if you build it up too much you will not be able to put the deck back. I will try to take some pics and put them up. Thanks for the help