If you did not state that you accept change of mind returns (generally indicated by selected 'No Returns' in the policy section), and your measurements were accurate, you're not obliged to refund.

From looking at your other listings, you have stated "7 day merchandise credit" on the ones I looked at, which is technically allowing change of mind returns even though you've qualified it with only for not as described - a buyer is entitled to a refund, not just store credit, if an item is not as described regardless of what you say in the return policy details, or whether you say you accept them, so the return policy is more to outline what you will or won't accept for instances of change of mind.

So if that was on the listing for the shoes, the buyer could expect it to be honoured.

Personally, and any technicalities aside, in this instance I would refund purchase price on return of the shoes in the same condition as when sent.