I'm 6ft tall, and thinking of getting a specialized langster. Dunno what size, i'm unfamiliar with the frame size. I have a Specialized rockhopper with 19 frame. I'm just getting back into biking after 2 years. Interested in picking up a fixie.

Nobody can really fit you over the internet. But you should probably be looking in the 56-58cm range.

Check your LBS and have them fit you.