The subject says it all. Can I file my US tax return on A4 paper, or do I have to track down 8.5x11 paper? (Before anyone asks, this return has to be filed on paper -- electronic is not an option.)

I have done some of ours on A4. I don't know if they scan the bits or enter by hand. To be on the safe side, I do not scale the form to fit a4, so the line spacing and etc is the same as if printed out on letter sized paper. I don't know the definitive answer. I have letter sized paper but it's all in fluorescent colors. I'll go with nice white a4.

once filed on A4 you could then fold into aeroplane and send it airmail...no stamp needed

I've filed my last two on A4 (rescaled to fit A4 as I did not bother checking this on the printing options). IRS had no trouble processing and sending me my bill.