To turn pro you don't need to be that big. You need to have size of course, but also shape and symmetry. If you notice most of the guys who are succesful pro's today weren't that big as they are now as they were when they won their pro card. Just work on that size a lil more, if you have good shape (or you are marketable with a great face, and look), look really crisp, have nice muscle bellys and have the posing, color, and overall look of confidence and polish, oh and a shit load of luck too (oops, almost forgot that lol) then yeah hope and pray you get lucky. Pay your dues.
But in all reality, it is a lot of luck. With so many quality guys out there now. It sometimes comes down to something simple as one guys color was off, while one was polished perfect. Sometimes you may come in peeled, while others you may be slightly off. Bodybuilding is a subjective sport. There really are a lot of injustices as Lou Ferrigno once said.
If it's in your heart, give it your all while you are young, but don't lose site of other things in your life. Just don't give up and try your best. That's the best advice I could give other than the usual be consistent with your nutrtion, training, rest etc.