I discuss designing for a #10 sized envelope in the ebook Your New Business Card, but wanted to reference other envelope sizes. The #10 sized measures 9.5” x 4.125” which is the standard business size, but some businesses might want branded envelopes in additional sizes. For example envelopes might come in handy for announcements, catalogs, and even window envelopes for billing.
This site has a great reference of envelope sizes http://www.designerstoolbox.com/designresources/envelopes/.
It shows a visual, gives standard dimensions and even available variations. If the standard is not the best option for you that is ok. You can choose a size that is most appropriate and apply what you learn to those dimensions. In choosing an envelope I want to remind you that what you plan on fitting into the envelope must be smaller than the actual dimensions of the envelope. I know it is common sense but a detail easily forgotten.
Envelope selection should also take into consideration additional costs for postage and slightly higher cost for custom printing. If it fits into your business needs then it is worth the additional cost.