These are equivalent to mail lite/feather-post size J - the internal measurements of each envelope are 290mm x 445mm. Classed by Royal Mail as 'Packet' size. 500 x Size J (6) Jiffy Aircraft Padded Bubble Envelopes.
500 x Jiffy Airkraft Size 6 (J) Padded Envelopes 290x445mm[5055502316178]