Simple Delivery only approximates the weight, so it's supposed to be OK if the item is bigger than it says on the label. eBay have said:
'If you use the recommended parcel size and your item exceeds the maximum size and weight of the label, you won’t be charged any additional costs. You can still use the Simple Delivery postage label. T he courier will accept your package providing it remains within the maximum weight and dimensions for their service.'
Personally I wouldn't risk it. If I have an idea the item is bigger than ebay recommend I would change it to the larger parcel size.