If you go their website, in the " Build a Boat" section and build the exact boat you have I think the information is there.
My tank size depended on hull choice and options.
Here is the link to the build site for 23 GPD .The size of your fuel tank , will depend on which pontoon set up you have.
If you have the standard set up ,25 Gallon Fuel Tank, 23.2 Gallon Usable Capacity