I am 5`8", 170 lbs with chest 39". Which size should i get Small or Medium? I need some room for layering (base and insulated) and I'm worried that size S will be too tight. Although size S is by sizing charts for me.
What would you recommend? Thanks

i would say you're prolly safe with a small, and medium if you wanted extra layers to throw in there...

I am 5'8 185 and the Medium was snug for me in the chest. I went with the Large and it is little long in the sleeves but not a big deal with the velcro cuffs. I think you may be able to go Medium but Small seems like it will be too small.

i'm 5'9" w/ a 42" chest, i needed the XL just so it would be long enough. this is normal for a jacket with this type of fit. Sleeves are perfect (without being cinched down they end where my fingers start). It is definitely not too baggy, Arc'teryx did a great job in the construction of this jacket to eliminate bulk in the front. i can fit my capilene 2, a heavy fleece, and a puff vest/jacket under it without any problems (this is my winter shell after all). I'd say start with the medium

I have a 40" chest and the Medium fits like a dream! But I am also 6'1" so I'm a bit taller. The Medium will give you the extra room for laying; the Small might be a bit too tight in the chest with a fleece under it. I guess it kind of depends on how you want the rest of the jacket to fit. You being on the shorter side I wonder if the sleeves length and total jacket length of a Medium might be too much. Not that it is going to drowned you, but half the point of buying an arc'teryx jacket it for that famous fit. Lucky for you BC has an unconditional return policy, so buy the Medium and if at any point you decide it is too big send it back for the small, no questions asked! Another option is trying it on, I always recommend trying things on when in doubt…