'You 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.'

I didn't know you could change the parcel size at no extra cost. Is this a relatively new bit of Simple Delivery tweakage? There is quite a bit of difference between the price of a Royal Mail 2kg small parcel, and a 2kg medium parcel. Strange times are these... 🤔