Hello '123.grandchildren4',
These Q&A boards are for other members just like yourself to show up and swap thoughts.
No one from ebay will appear here, and even if they did, they are not able to cancel your
purchase.
Only your seller can do that for you, so you must very sweetly ask the seller to cancel the sale.
As you know, you should have sorted the size thing out and asked any questions before bidding
and buying. The seller does not have to cancel, and can instead file an Unpaid Item case
against you if you do not pay. Failure to pay will result in a Strike on your ebay record which
lasts a year.
If the seller agrees to cancel, she will send a request for Cancellation to you, -- so the second it
arrives just click to respond then click to agree. As soon as you agree to cancel it is done and
you will no longer have to pay.
Your only other choices are pay, or don't pay and get a strike if the seller files the Unpaid Case.
In future, always ask any questions before bidding and buying.
Good luck 