Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Where does the wrong size message appear? on the printer, or on the computer screen?
Are you printing from MS Word? if so, check that MS Word's printing settings, and the printer's driver settings on the computer, and the printer's own settings plug your choice of paper tray, are all correct.
Note that if you set the envelope size to e.g. 9 x 3 inches, it may have to be set to 3 x 9 inches instead... or the like. Getting the order of the two dimensions wrong can do that.
I regret to have to say, I do not know MS Publisher well enough to advise you, so I have Opted Out, giving up the case to another Expert.