I’ve been printing for 30 years and still get confused about paper and envelope sizes. I could go into a lengthy explanation of how paper gets its “weight” and what the difference is between an A2 and 5.5 Baronial Envelope, but it’s a lot easier to just look at a chart and pick out the one you want. I’ve attached a PDF that gives you that basic information to help communicate more clearly when you’re ordering something from your printer.
» Envelope and Paper Size Chart