He has a cattle shed in his backyard that is 15 m ˣ 6 m in size. He has a cattle shed in his backyard 15 m ˣ 6 m in size. He has a 15 m ˣ 6 m cattle shed in his backyard. I like the 3rd choice. I would also add the hight.

its not wrong to say it that way i belive, but i think saying "dimensions" jsut adds specification to someting for example, what are the dimensions of the fish tank? - answer "its 10L x 10W x 20H inches what size is the fish tank? - answer "its a large one" its very common for us to say "what size is it?", or "how big?" is it instead of asking" what are the dimensions?" so i your sentence its fine to say its size is...