Did you have an opportunity to try it on in the shop (ie - did they have changing rooms)? If they did you have no recourse as it really means you just didn;t like it. As it is they are offering you an exchange, which is more than they have to.
I could be wrong, but there might be some recourse if they had no changing facilities (vaguely recall something about this, but 99% of clothes shops have them these days), but I would reckon it no more than n exchange...which they have offered. Is there anyone you know might like something from the shop & you could sell it to them?