Hi Helibuff,
The openings are: 20 x 36 cm
Inside the storage box is: 40 x 56 x 17 cm
The dimensions of the were controlled for me by the space I had between the metal bars of the trailer. I wanted to keep the openings relatively small so as not to weaken the floor too much. The boxes were constructed using the same epoxy fillet and taping approach as the rest of the trailer and attached to the underside of the trailer using fillets and tape on the outside only. It was too tricky to apply fillets on the inside of the box where it joined the teardrop floor.
Hope this helps.