Paper size for tax forms

This might sound like a nitpicky question, but do the IRS and the various state tax authorities accept forms printed on A4 sized paper? Has anyone had any issues with this in the past?

I'd hate to have a form rejected due to wrong paper size, but also don't want to have to hunt down US letter sized paper here unless it's really truly necessary.

Thanks!

I filed taxes last year for 2008 (I'm a UK citizen, but worked in the US in 2008) and I sent forms that were printed on A4 paper... they were processed with no problems at all and I got my tax refund a few weeks later .

I've filed A4 sized paper returns with the IRS for decades and never had a complaint.

I have filed hundreds of tax returns on A4.  It is absolutely fine.