waste tank size/location
I think you need to shoot for 20 gallons, but it sounds like you won''t be able to do that given your dimensions. All you can do is go as big as you can.
I would build a very sturdy platform over the tank. Anything that might cause the tank to rupture must be avoided at all costs. It would take a long time to get the smell out of your boat.
Buy the best hoses available. They are expensive, but worth every penny as odor is a problem you want to avoid.