what is the difference between dimension and size?
in my transklation I have such kind of sentence: he has cattle shed in his backyard, which size is 15 m ˣ 6 m ( is it correct to write in this way?) thanks

Question

Answer

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.

Answer

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...

Ne manquez pas cette occasion d'apprendre une langue depuis chez vous. Découvrez notre sélection de professeurs de langues expérimentés et inscrivez-vous dès maintenant à votre premier cours !