If you are using Word, start a new document, go to the Tools menu & select Letters & Mailings, then Envelopes & Labels. There you can type in the return/delivery address (depending on which one you are working on) and then click the Options button. This is where you tell Word what size envelope you are using. You can create a custom size envelope if one of the options doesn't work with your particular size envelope.
After you've done the above, click "add to document" - the appropriate size area for your envelope should display. Now you can change the font, center it, whatever you need to do.
When you are ready to print, go to the File menu, Print...before you click OK, click on your Printer Properties and tell the printer what size you are printing on. Again, you can create a custom size if necessary.
I hope this helps. If not, post back!