Secutiy Envelopes Expandable up to 2". Opaque for confidentiality. Double glue strips and double perforations at the top let you reseal these envelopes for another use. Made of waterproof, co-extruded polyolefin. Lightweight envelopes are durable and will save on postage costs 100 envelopes per box. Letter/Medical Record 14-1/2 x 11-3/4 Medical File Envelopes