A "normal" ocean-going Ro Ro ship would be over 200m long and quite a bit higher. Typically only the lowest deck can carry trucks and heavy loads and the upper decks carry cars. Their solution of only having the heavy load deck is sensible since they need a low center of gravity to counteract the sails and any added deck height hurts the sailing characteristics. With all that said, there is not much to scale it up. They could make it a bit longer but probably mostly easily: risk in bad weather, structural integrity demands from the added sails and all that may quickly add up.

I did some searching around for this (not a subject matter expert).

Basically the sail area grows with length squared, but ship mass and resistance grows roughly with length cubed. So propulsion gets weaker with size.

To move a full sized freighter, you would need a mast the size of a skyscraper and we don’t currently have materials that can support that.

If we did have a material that supported sails that large, it would still be a problem because you are functionally making the ship top heavy (when wind is applied) increasing how likely it is that the freighter rolls over.

It scales horribly. Essentially, as the ship gets larger, the sail area has to be even larger (proportionally) to maintain the same speed. Further, the larger the sail, the more susceptible to damage and the harder it is to control.