Note that the small, medium and large specifics can vary per category.
If you go to the postage section while drafting an advert it will show some guideline recommendations, but it you click ‘see sizing and compensation details’ you are told the size & weight limits of each option
Generally the sizes are as per each carrier, which means you may or may not fit to the buyers chosen option if you’re at the limit of some
… and for weight I’ve found that most often small is 500gram, medium is 1kg and large is 2kg
Do beware of the Royal Mail options, as the buyers postage is paid to you and you buy the postage, whereas other carriers are paid to Vinted and they generate a label for you
For packaging I recycle most of the time, and also add a bin bag plus lots of tape. (I don’t want breakages or burst packages in transit)
Most parcels I receive are kept, then say Lidls boxes, boxes from friends shops etc
I also ‘invested’ in dedicated old up Royal Mail size large letter boxes. On eBay sales I have often made my own to keep things within Royal mail dimensions, but ‘proper’ boxes work much better
An Amazon ‘small’ card ‘envelope’ can be good but they sometimes bow and go over the large letter depth in the middle
Lidl vegetable boxes are good to cut to size but have quite some weight
The problem with boxes is when you’re at a weight limit you’re taking up precious weight, so I have cut up a few boxes to make them as small as possible
If you’re within the Vinted weight limit then I’ll put in as much packaging as possible
Once I’m happy that the item is safe I finish off with a black bag wrapped around, plenty tape then the label
As I put on a lot of tape I must at least think of the recipient, and make sure that they aren’t going to need to hack into the package and cut the contents. So I may have a heavily secure outside with the box/cardboard only basically stuck down. That way any hacking open is only the outter layer