There are names for the different sizes of paper. In different writing paper sizes of the world, different sizes and names are used. There are also envelope sizes made to fit different kinds of paper.
In the United States and Canada the normal size of paper is called letter size, or sometimes US letter. The other common size of paper in the US is called legal, or sometimes US legal. It is similar to the size called foolscap in Britain. It is sometimes used for writing personal letters to people.
There is also a size called tabloid. It is 11 inches wide, and 17 inches long. That is the same size as two pieces of letter size put next to each other. There are also many other paper sizes used for writing notes, for big posters, for books, and for other things. In Europe and other places an international system of paper sizes is used. The normal size of paper is called A4.