Problem buying online

[Frigiliana]

Been thinking of buying this jacket online , i just can't work out the sizing , i would say I'm a medium and have a jacket marked medium and a few shirts the same , but i asked my wife to measure across my shoulders and it measures "18 so according to there sizing chart i would need to order a Xlarge , has anybody got advice on this subject?
this is the jacket, Stream jacket by Woolrich
http://www.contextclothing.com/item.php?id=2142

 

[LukeM]

Lay out your best fitting SC and measure that