Gros chien vs Chien gros
How can you tell whether the adjective comes before or after the noun?
Most adjectives come after the noun. There are some, however, that come before the noun. These are adjectives that refer to Beauty, Age, Goodness or Size (BAGS is an easy way to remember them).
In your example, because "gros" refers to the size of the dog, the adjective comes before the noun.