A typical 100 gal. aquarium would be ~ 24"x 48"x 18" (HxLxD). Obviously you could make it any dimension you would like to fit your space.
Not sure what you are going to use this deck
box for but my first inclination would be to built to fit your specific space. We recently built a deck/dorade box combination out of Coosa to fit our space, then glassed the outside. Very strong and one can stand on it.
Again depending on your intended usage, you could use structural PVC panels
(or other materials that do not absorb water
like plywood) and glass them to make your custom box. This allows you the flexibility to build the size you want and will be less expensive than buying
a commercially built box.