Replying to Length and binding placing

I had bought my skiis at a shop and the guy there was taking care of everything i needed to know.. but im just not shure. I am 5'-11" now and I only weigh about 125. what length should I be using? And also, I was interested in my binding placing. The guy at this shop did it for me of course, but it didnt seem right after I watched all these videos of freeskiers. It appears like the bindings are more centered, where as mine seem to be far back like normal skiis. does it even matter?