A buyer can ask a seller to cancel an item, but they do not have to oblige.
Sometimes, even if the cancellation request is done minutes later, an order may have gone through to the seller's warehouse as a sale, and too late to stop the process for dispatch.
If the seller has a Returns Accepted policy, they should do if they are offering items in different sizes, then you can return the item for the reason... wrong item ordered / doesn't fit , for a refund, but you will have to pay for the tracked return postage.
@hayley0504