At least for an individual listing we can specify the size and weight. But ebay have no idea how to combine postage when buyers buy more than one auction. 

I sold 3 auction items to the same buyer. Each 1kg and a 35 x 45 x 16cm parcel and ebay combined that into a 3kg parcel with the same size!  

In reality all three weighed less than 1 kg but they would not fit in the same small parcel as one.