Normally you have to return the item at your own expense to get a refund of your original payment.
Let your seller know the problem.
Do the measurements of the shirt match the measurements given in the listing? If the only measurement given in the listing is Small/Medium/ Large, does the label in the shirt match? In those cases, the seller sold you what you bought. If they do not match, the shirt is 'Not As Described'.
Stay polite and friendly with the seller. This will always give the best result.