These are equivalent to mail lite/feather-post size H - the internal measurements of each envelope are 260mm x 345mm. Classed by Royal Mail as 'Packet' size. 500 x Size H (5) Jiffy Aircraft Padded Bubble Envelopes.
500 x Jiffy Airkraft Size 5 (H) Padded Envelopes 260x345mm[5055502315942]