Now, you will want to specify envelope size. Click the Options button on the Envelopes and Labels dialog box. Open the Envelope Options tab.
Specify the size of your envelope in the Envelope size drop-down box. If the correct size isn't listed, select Custom size. Word prompts you for the dimensions of the envelope.
You can also adjust the margins for the return and delivery addresses. Use the selection boxes in the appropriate section to adjust these margins.
Once you’re done specifying your options, click OK.