Proper paper size for manuscript submissions

A reader writes to ask:

I have just one question concerning the "1in from margin" factor: what page size is normally used?

8.5x11?
6x9 ?

I see the '1in from margin' statement everywhere but nowhere I've found do they say the page size.

In the United States, Canada, and a few others places, print and submit your manuscript on white standard letter size paper, which is 8½ by 11 inches. In most of the rest of the English-speaking world, including the U.K., Australia, and New Zealand, use A4 paper, which is 210 by 297 mm, or about 8.27 by 11.69 inches. (A4 is part of the ISO 216 standard, which makes a lot more sense than our system.)

But really, when have you ever seen a ream of 6 x 9" printer paper at Staples? It's a fairly standard size for book publishing, yes, but that wouldn't make it at all practical for manuscript submission.

By the way, before you ask, if you're submitting a short manuscript on paper you'll need to buy 9 x 12" envelopes.