mens rab infinity down jacket

It's going to come down to taste really. The rab infinity is designed to be an athletic cut so if sized "properly" it should be fairly close to your chest after wearing normal winter layers (an underlayer and light or midweight fleece). A lot of folks though like their insulating piece to be be a little roomier so they can throw over all their active clothes (base layer, fleece, maybe a thin vest or puffy layer).

As long as your jacket isn't tight (compresses the down so not as warm) or so baggy that you can't seal up the waist, writs, and head/neck region you should be ok.

Is the infinity box wall or sewn through?

The infinity is sewn through. At 5'8" and 200lb I would say the Large would be the right fit for the OP. I had one in this size a couple of years ago, and when I dropped from 195lb to 175lb I felt the need to downsize to a Medium. I'm 5'9" for reference.

I feel as though it fits well on my shoulders and chest area with enough room for inner layering fleece etc. But, I can feel the arms are a bit to long and I know they are supposed to be but just a bit to long.