Yo should be able to change size (but not weight).   Click on Get Prepaid Label and under the heading Package details click on the pencil icon and you should get:  No extra charge. Changing your package size won’t cost you anything.

If the weight is wrong you can cancel/refund the transaction with the buyer's agreement via the dropdown menu arrow for the item on your orders list.  Then relist with correct postage for the buyer to buy and pay again.

The alternative would be to buy and pay for the postage yourself outside of ebay.

It's always been best to get your parcels ready for when you list so you don't under or over charge on postage.