John, nice post. Here are two more good links on the differences, including sizes. Good idea to bookmark them:
http://pe.usps.com/text/dmm100/what.htm
http://pe.usps.com/text/dmm100/postage.htm

Ironically, I just had a small envelope returned to me by the post office, stamped "postage due."!!!
Just a day or two ago, I said (posted here) I never had problems!! Go know! I am sure they made an error. I think they reclassified it to "large envelope (flat)."

Robert