I'm not an expert, but....
The envelope industry is annoying because they use the same size
numbers for different standards. For example, a Number 8 white paper
envelope is very different from a Number 8 manila clasp envelope.
However, for our purposes (ordinary white mailing envelopes), the size
number is the approximate width of the envelope in inches. The most
common "small" envelope today is a 6 3/4, which is about 6 3/4 inches
wide. The "legal" or "business" size is a number 10, and it's about ten
inches wide (actually about 9 1/2). The same would apply to government
stamped envelopes of some years ago -- a #5 is five inches
(approximately) wide, a #8 is 8 inches, and a #13 is huge, more than a
foot wide.