178mm x 254mm Board Back Envelopes - Please Do Not Bend (Qty 125)1 2 day delivery as standard on this product 178mm x 254mm size manilla board backed envelopes, which is an oversized C5 envelope size so if you're finding a regular C5 just a little too tight for your contents, then this is ideal. Heavy duty 120gsm manilla paper to front printed with 'Please Do Not Bend' and hard back for support and protection to the envelope contents. Envelope size is 178mm x 254mm with a peel & seal open close on the short pocket