I'm 6' 2", 210# and I just moved from PA. I spent the last winter at an even smaller mountain (Ski Roundtop) riding on a Concept TMS 164. I was very happy with how it performed on groomers, and it's good for jumps and boxes, but I wouldn't really bother with any rails on it.
Seven Springs has really focused on building their terrain parks and other features because, as you said, they don't have long slopes, gladed areas, or any sort of backcountry. Mountains that are much, much bigger than Seven Springs don't have two halfpipes like they do. I think that is probably the biggest reason why people there are riding shorter boards. Their focus is toward park and pipe riding.
The problem with riding a shorter board is that you also have a shorter effective edge, so your weight is being distributed over a smaller area. Less contact with the snow/ice = less grip. This is why you see things like "Magne-Traction" and "Pressure Distribution Edges" and "Vario Power Grip" on boards these days. The idea is to increase the amount of contact your board makes with the snow.
I would never consider riding a 156. I don't have any confidence that a board of that size could be stable under my weight at any speed. I was shocked when I read that they don't carry boards bigger than that. Apparently they don't care about anyone who doesn't want to ride fast or weighs more than 170lbs.
All that being said, you should see if you can demo a longer board to see how it feels. It will require slightly more effort to turn, but it should also hold on the ice a lot better. If you don't like it, then stick with something short and look into boards with non-traditional sidecuts like Magne-Traction and Vario Power Grip. Everyone seems totally gay for Magne-Traction right now, but I'm not sold on it until I ride it myself.
The bottom line is that you can look at charts all day and take advice from a thousand sources, and you may only end up being more confused than when you started. Try and ride as many different boards as you can, and go with what works best for you. That's all that really matters.