Not having used an envelope yet I feel glad that I found an a size that is perfect for my purposes. I have books that are 8-1/2" x 11" and can fit two per envelope. Even if the clasp is weak or I must reinforce the glue flap with tape I can take the extra measures. The capacity of the envelope matters most.
Great for organizing year end tax papers receipts. The envelopes store up right take up little space. I lable them with easy identity. They are strong and last for years.