Sellers don't have to agree to cancellation requests.

If yours has been refused, you will simply have to wait until the item arrives, then open an "item not as described" case under eBay's money back guarantee. Be careful how you describe the problem - don't say it doesn't fit, make clear that the size received was not the size advertised.

I take it you're familiar with eBays money back guarantee - there is alink to it in every listing. Or see:

https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041

https://pages.ebay.co.uk/ebay-money-back-guarantee