[xmcp123]

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.