The people who invented the A series of paper (A4, A3 etc) deserve a medal. Best thing since sliced bread.* I'm working on some illustrations at the minute, and we're not quite sure how big they'll be printed. But it doesn't matter. Because all the paper is the same proportion, we can do them 71% smaller and they'll still fit exactly.
*Why is every good invention always compared to sliced bread? Yes, it's useful, but personally I think disposable toilet paper wins hands down.