The precise measurements and specifications are floating around out there on the internet - it's been a while since I came across it - the one thing that I was amazed at was how small the lifejackets were. I dummied up one here a short while ago and was a little suprised as to how small the lifebelts were. In comparison, it's comparable to an adult small or a child's large ski jacket of today.
My notes have the lifebelts at 23 inches from top of shoulder to the bottom (or 46 inches if the jacket was laid flat and open, a foot across, and 2 1/2 inches thick.
Cameron's Titanic lifejackets were visually well-done, but IMO A Night To Remember's recreations were more spot-on (the ones used for deck scenes - not the pillowy ones made for the stunt jumpers)