Getting the biggest guitar is usually a good way to a big sound, but you have to look at the woods and style of guitar too. A big body, with a sitka Spruce top wil be loud, or at least it should be if the company have built it correclty, thats usually a big IF. And if you find one that sounds great, and just right for you, don't wait and order one of the same model, becuase it will sound different.
Joe