Thank you for answering.
I'm not very muscular so I guess my shoulders are pretty small. When it comes to shirts I usually use custom S or M slim fit (PRL). The problem with sweaters in size S is that they risk getting a bit to short. As I said I'm 6'. Do you reckon a S J. Crew would be long enough :)?
I haven't bought anything from J Crew for a while so I don't now for sure. Sorry.