The name of the game here is presentation. What you are really designing is packaging. How do you make your envelope stand out in a pile of mail?
Like a letterhead, envelopes say a lot about the company. An envelope’s message is shaped by its color, size, format, type, graphics, seal, contour, paper and texture. They may be minimal or elaborate, simple or sophisticated. How we respond to them is determined by the design.
There are many standard sizes. While a printer is capable of producing any size and shape, keep in mind that it’s much more cost effective to use a standard size. Consult your printer to get a list of existing sizes of envelopes that match your paper stock. Envelopes that are already folded and glued will be much less expensive than envelopes that need to be converted. The only draw back of using already converted envelopes is that you must keep printing at least ¼-inch away from the edge. There are a few other considerations. To qualify for standard postage prices your envelope much be a rectangle, it must be at least 3-1/2 inches high by 5 inches long and 0.007 inches thick and no more than 6-1/8 inches high by 11-1/2 inches long and 1/4 inches thick.
The last, but certainly not the least design consideration is the area that needs to stay clear for the mailing address, postage and routing codes, consult USPS for exact specifications, based on the size of your envelope. Or take a mock up of your design to the post office before you print it.
Of course, your design solutions should maintain continuity with your other corporate identity pieces, but don’t be afraid to give it some flare and push the boundaries.