How to change the default paper size in LaTeX

The default paper size coming with every latex system, including MikTex, teTex, Mac versions is A4. If you want to change it to letter, please follow these steps,

• If you are using an old version(not pdfetex), or, you are still using dvips, dvipdf to create ps(pdf) files, you need to change the following files.
  For dvips, edit $TEX/texmf/dvips/config/config.ps, find
  

  @ A4size 594.99bp 841.99bp
  @+ ! %%DocumentPaperSizes: a4
  @+ %%PaperSize: A4

  you can either comment them by adding % in front of the lines, or move these lines behind the letterSize section.
  For dvipdf, edit $TEX/texmf/dvipdfm/config/config, find
  

  % Set default paper size here
  p a4

  change a4 to letter
• If you are using pdflatex, depending on the specific version of TeX, you will need to edit either $TEX/texmf/pdftex/config/pdftex.cfg or $TEX/texmf/tex/generic/config/pdftexconfig.tex. Change pagewidth to 8.5 true in; pageheight to 11 true in.
  However, I found for some systems, manual changes don't work! In these cases, you should run texconfig (or similar configuration program in MikTeX, gwTeX) to change the default paper size. It will create a personal configuration file in your home directory.