Well, legal paper is 8 1/2" X 14". The question is whether the document being sent to you will be folded or lie flat. If’ it’s to be folded, then a standard number 10 envelope (4 1/8" x 9 1/2") will work. Otherwise, you need an envelope at least as big as the one recommended by the USPS.
unless you send the biggest size of any answer you get, then you migh ask someone in the New York government what they require. send them an email or give a call.
I found that on yahoo and it’s not my own, but it seems legit. I work at a retail call center selling office supplies, and I would confidently recommend a 4 1⁄8 x 9 1⁄2" #10 Gummed White Business Envelope. I won’t post a link to any vendors that carry this, as this is a message board and I don’t want to be breaking any rules/ advertising anything, but a simple google search will help you find this size.
This is also helpful advice for sending an SAS Envelope:
What I’ve always understood it to be, is: Legal size will fit a standard sheet of paper folded into thirds. A letter-size envelope is smaller, and will fit a sheet of paper folded in half, and then in thirds.
What some people are calling a legal envelope (no. 10) is a business envelope. A true legal envelope would be much bigger (I think the largest size noted above.) It’s quite possible that they meant business instead of legal though. Can you contact someone or check a website to make sure? If you know what they’re sending back, that should give you an idea too.
That page is for flat rate so it’s a special envelope that fits that size or smaller. For standard weight and size-calculated shipping, the category the USPS called “letter” ranges from 6.125x11.5x0.25 to 12x15x0.75. That last measure is a thickness and the envelope can of course stretch thicker but might bump you up a category. As you see on the second link, the priority envelope is $20 so you can normally send for much cheaper if you don’t have any specific requirements.
They mean a #10 envelope. Forty years ago, there were no home printers or Office Depots. Most households had a box of #6 3/4 size envelopes from the drugstore, used for sending letters on note paper to distant relatives, and had to look around for a #10.
This sounds right to me. If someone is asking for a stamped and self-addressed envelope, it’s for the purpose of sending a 8.5 x11" sheet (or two), folded into thirds, back to the sender. That purpose is best served by a #10 envelope (the 4.25 x 9.5" one).
A request for an envelope that would hold actual legal-sized paper unfolded would be much more specialized (and unusual); such a request would specify the value of stamps that should be placed on the envelope, which presumably would be more expensive than mere first-class standard postage.
