One maximum since October 2014
Royal Mail changed how parcels are treated for people and businesses that send fewer than 1,000 parcels a year. Our changes 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. The two previous options were 450 x 350 x 80 mm and 350 x 250 x 160 mm. A box a few millimetres over the current maximum is a standard parcel.
Rolls are a separate exception
The same page gives an exception for roll or cylinder parcels that can still go as small parcels: maximum length 450 mm and diameter 80 mm. A tube outside that is outside the exception.
The page also says sending a small parcel on that network costs less than sending a standard parcel. The box ranges named for that limit are on the postal packaging guide.