The published maximum
Our changes page, for people sending fewer than 1,000 parcels a year, gives one small-parcel maximum since October 2014: 450 x 350 x 160 mm on the outside. The previous options were 450 x 350 x 80 mm and 350 x 250 x 160 mm.
A Packaging Suppliers mailer that finishes over that outside size is a standard parcel. The page says a small parcel costs less.
Bags and rolls
Grey mailing bags are opaque polythene for soft goods, described as recycled film and recyclable. They are not a rigid box, so the finished parcel is what you measure, not the flat bag.
A roll still has its own exception: 450 mm long and 80 mm in diameter. Kraft rolls and postal tubes are not the same rule as a mailing bag.