'I thought ebay were responsible for any errors in weight and size if the seller accepted the recommended postage.  But when listing it says you should make sure you get the weight and size right or you will be charged.'

You're not the only one finding this confusing... 😖

What it seems to mean is that if you let SD 'decide' the weight/size, then even if SD gets it wrong, the postage label can be used, and eBay will sort out the difference. But if you put in your own weight/size and get it wrong, then you will be responsible for the difference and may be charged for the underpayment.

What it is boiling down to, is that SD doesn't want us to do anything for ourselves where postage is concerned... even messaging between buyer & seller is now being 'taken over' by SD.