A pretty standard size is #10, which is 9½" × 4⅛". Fits US letter (8½" x 11").
Minimum envelope size for mailing is 3½" × 5" (and 0.007" thick) according to this USPS article from 2007. Actually, a better source is this “Sizes for Letters” post from USPS.
Looking at USPS’s website, it seems like the “No. 6¾” envelope is a typical “personal” size. Wikipedia says the dimensions are 3⅝ × 6½. I guess the personal envelope would fit a junior paper folded in thirds (2⅚ × 5½), though I’m not sure people do this.
I’m kind of interested in the “A2” size (4⅜ × 5¾) because it’d fit a US letter paper folded twice (which is a “junior” paper folded once “hamburger style”). Wikipedia says this is called “Lady Grey”, but I can’t really find anything on the web that refers to this. Maybe it’s a term in other countries? Searching USPS.com for the term in quotes returns nothing. Probably my best bet is to use the more typical No. 6¾ envelope.
… Okay well looking at Patrick & Co.’s online shop, it seems like these A2 envelopes are “Greeting Card/Invitation Envelope, A-2”, so maybe they are used.
Crane Stationery “Kent” (?) envlopes are 4⅜ × 6⁹⁄₁₆. Some other ones I saw at Patrick & Co. in SF (November 2023) were 100mm × 141mm (3.94" × 5.55"), and these corresponded to notes that were 3¹⁵⁄₁₆ × 5⁹⁄₁₆. Another one I saw was roughly the size of the Moleskine in my back pocket, which is roughly 3.5" × 5.5".
Card envelopes
Trying to look at USPS’s online store (USPS online store, though this link doesn’t work too well), I can’t see a uniform greeting card size. Seems like it’s generally around or over 5 inches for the width, and 7 inches for the height.