The outside of the board
For senders of fewer than 1,000 parcels a year, the small parcel maximum since October 2014 is 450 x 350 x 160 mm on the outside. The older limits were 450 x 350 x 80 mm and 350 x 250 x 160 mm.
A Packaging Boxes carton that suits the goods but exceeds that outside size is a standard parcel. The changes page says a small parcel costs less.
Styles that name the limit
Crash lock boxes are 150K/T in six sizes the category describes as fitting Royal Mail small parcels. White postal boxes in stock are also described as qualifying. A tube uses the roll exception instead: 450 mm long and 80 mm in diameter.
The trade discount does not add millimetres.