"My dog is big."
Translation:Mi perro es grande.
I'm not sure what your question is. Adjectives often follow nouns in Spanish, so a big dog is "un perro grande" and a black cat is "un gato negro".
Now, the sentence here is "Mi perro es grande", which is "my dog is big", and it never has the word "big" right next to the word "dog", so the question of whether the adjective comes before or after the noun never comes up. And if you want to say "my cat is black", it would be "mi gato es negro".
Again, I'm not sure what you were asking, but I hope I've answered at least some of it.