If you're really that concerned, you can contact eBay's customer service using the chat feature. Give them the item number, and ask them to read the messages. Explain that the buyer stated their was a problem and implied they wanted to cancel, and you have not heard back from them so you cannot resolve the issue for them.
Then ask for instructions on how to cancel due to the buyer asking to cancel. They will likely give you permission and walk you through it.
eBay sometimes asks buyers to confirm whether they actually asked to cancel. The only way you would have an issue is if the buyer wrongly states they did not ask to cancel. By speaking with eBay chat first, you at least have a record that you were given the go ahead by a CSR to cancel.
With that said, this is kind of overkill. In a scenario where a buyer says this isn't actually what I want, then ghosts you, you almost certainly are fine to cancel. You can send a polite message to the buyer after cancelling that says something like "Because you stated you did not wish to proceed with the order for the item in x size, I have processed your cancellation. No further action is required on your part. The listing you purchased was for the item in y size, I do not have this item in x size so I cannot make that substitution. Thank you for shopping with me."
Alternatively, if they haven't paid - the easiest thing to do is just to wait 4 days and the 'Buyer Hasn't Paid' reason will come for cancellation. If the buyer ghosted you, and you still aren't sure, use that.
