i'm 5'8", 155lbs, what would be the best size? S/M or M/L? what about wearing it over a form-fitting motorcycle leather jacket???

Hey Jcr, You will want to go with a medium large. I am 5'10 155 and I personally wear the m/l and it fits me very well. If anything, a bit on the small side but I am not complaining. If you are going to wear this over a leather motor cycle jacket, you would want something that is going to offer a bit more room for adjustment.

I agree with the M/L size plus you'll be able to carry more gear like creature comforts.
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