With breeding,how big can a dog get?

another very very clueless backyard breeder

I think you're asking which dog breeds are the biggest.
The Irish Wolfhound is usually recognized as the tallest dog breed-- males are 32 inches tall or more at the withers (the top of the shoulders). Scottish Deerhounds are almost as tall.
The heaviest breed is usually said to be the Saint Bernard-- up to 180 pounds for the largest ones.

It might be possible to breed an even larger dog, but it is not regularly done. Giant breeds of dogs often have some health problems, such as (for example) hip problems.

Here's a rundown on 13 of the largest dog breeds:
http://www.vetstreet.com/13-largest-dog-breeds-towering-and-commanding?WT.mc_id=Large_Dog_Breeds

What? Genetics dictate the size of a dog.

You can't breed two Chihuahua's & get a big dog. ?????

You have to breed a big dog to a big dog to get a big dog.