I usually wear M (I'm 6' and pretty fit) but for Zara I have to go one size up and buy L. And even then some articles will just be too tight around the shoulders so yeah if you have large shoulders like me you might have trouble finding clothes that fit.
Their return policy is pretty nice though. I just go to the nearest store and present the receipt on my phone and they refund me no questions asked. DId it twice already. Never had any problem.