The standard size for letters (thus, the name) and letterhead, copy machine paper, and desktop printing papers is known as letter size. The actual dimensions of letter size paper depend on where you live. Letter size paper is often folded to create booklets and brochures.
North American sheet sizes are used in the graphic arts and printing industry in the United States and Canada. Page sizes are measured in inches. The most common sizes are multiples of the standard letterhead or letter size of 8.5 inches x 11 inches. It is slightly shorter and wider than A4 letter size paper. Tabloid size paper is often folded in half to letter size for newsletters.
Common sizes of sheets of paper used in the graphic arts and printing industry outside of North America are ISO Sheet sizes. Paper is measured in millimeters. The A4 size is roughly equivalent to letter size used in the US/North America. A4 paper is 210 x 297 millimeters, slightly longer and narrower than the North American letter size.