The size is set by the carrier
Royal Mail changed how parcels are treated for people and businesses that send fewer than 1,000 parcels a year. The detail is on our Royal Mail changes page, not in a guess at the counter. That page says two old small-parcel maximums were merged in October 2014 into one overall size.
The current maximum it publishes is an external measurement: 450 mm long, 350 mm wide and 160 mm deep. It also says sending a small parcel on that network costs less than sending a standard parcel. A box that is a few millimetres over the maximum is no longer a small parcel, even if it looks small on the bench.
What changed in 2014
Before October 2014 the page lists two options. One was 450 x 350 x 80 mm. The other was 350 x 250 x 160 mm. Those were merged. The page says we still stock boxes made to the older sizes, and that we stock the new maximum as well.
Rolls and cylinders have a separate note on the same page. They can still go as small parcels at a maximum length of 450 mm and a diameter of 80 mm. Postal tubes are the cardboard tubes with plastic end caps, for posters and documents that must not be folded.
Where those boxes are on the site
The Royal Mail changes page lists compliant sizes we stock, including inch sizes of single wall boxes, a white postal box, the 449 x 349 x 79 mm and 349 x 249 x 159 mm external sizes, and the 450 x 350 x 160 mm box it marks as the new maximum fit. It also lists postal cardboard wraps in C1, C2 and C3.
On the shop, start with Royal Mail small parcel boxes, Royal Mail large letter boxes and white postal boxes. The wider postal category holds mailing bags, postal boxes, postal tubes, document wallets and postal scales. Crash lock ecommerce boxes are a separate range that the category describes as fitting the small parcel guidelines.
Bags, wraps and the rest of a postal order
A rigid box is not the only postal pack. Mailing bags, including grey polythene mailers, suit soft goods. Jiffy and other bubble envelopes suit items that need a lined bag rather than a carton. Book wraps are the depth-adjustable cardboard wraps with a peel-and-seal strip.
The Royal Mail page says the compliant sizes it lists are held for next working day delivery. Across the shop, orders placed by 3pm are dispatched the same day, and UK delivery is free on orders over £50 ex VAT.