Don't open a dispute as it's a change of mind return. Send the seller a message and let them know it's too big and would they be interested in you returning it to exchange for a smaller one. You will be responsible for return postage costs and also the cost of resending the new item. If you tell the seller you are willing to cover all the extra postage costs, they might be more willing to exchange.
If as Lyndal said they don't accept returns, then they are not obliged to accept the return for exchange. You could still ask though, but don't get upset if they say no.
