How Do I Add a New Paper Size?
Envelopes come in tons of different sizes. Printers are generally capable of printing in any of these sizes, but computer software only has a few of these sizes listed in its "paper sizes" options. If you want to print something other than #10, B5, C5, DL, or Monarch, you'll probably have to add your envelope size to your printer's computer software yourself. This is a bit unintuitive, but if you do it once, your custom size should remain an option in the future.
The first step to setting a custom paper size is to get to your printer's software's options window. If you're printing from your browser, this means you have to first hit "Print using the system dialog..." and then click the "Preferences" button. This will bring up your printer's proprietary setting screen. You'll then want to navigate to the paper sizes and press something like "Add Custom Paper Size". Usually "Paper" will be a separate tab, but you might have to go to a detailed settings page or select "New Paper Size..." from a paper dropdown menu. Once you open the window though, you can just type in the dimensions of the envelope, save it, and you're good to go. If you don't know the dimensions of an envelope size, Envelist will display them when such envelope is selected.
Another possible kink is the process is that if you are using an HP printer, you can't set a paper height of less than 5 inches. The way to work around this is to set your envelope's height as the paper's width, which can go lower, and to set your envelope's width as the paper's height.