I just purchased a Mountain Hardwear Sub Zero SL and I'm not sure if it's the right size. How are these things supposed to fit? Is there anything I can look at in the way this jacket fits to indicate if it is the right fit?

This jacket is cut to fit over all your other layers. You can size it down probably one size safely, if you are really after a trimmer fit, but this is a down jacket after all and it is still going to be pretty puffy. The original idea behind jackets like this is that you throw them on over your mountaineering outerwear when you slow down or stop moving to make camp, so they are made to fit over everything else you would be wearing. The only thing you positively wouldn't want is for it to fit too tightly. You would be squeezing the air out of the down at that point and the jacket won't keep you as warm. Hope that helps.

Not really. If you like the way it fits, then it fits. If it's way too sloppy or too long or the sleeves are too long, then it's too big. If you like it baggy it should be baggy. It's totally subjective, depending on how you want it to fit.