I'm 5'10", 155 pounds and I'm riding the medium frame, but I'd recommend contacting Mountain Cycle with the sizing questions.
I had a hardtail before and would have a sore back after long 12+ mile rides. Since then I've been doing situps and things to strengthen my back. That and the added short travel seatpost make for a great ride. To be honest I haven't even ridden this frame with a regular seatpost so I couldn't comment on that, but as long as you have a decent fork upfront the ride is fine.
Good luck building it up. I've had mine now for half and love it! It can really get up when you have it built light but it's still fun going off small/medium drops and is a great trail bike.