In the US US letter size is standard the rest of the world A 4 is standard A 4 is based on he metric system and is a rational segment of a certain size standard metric sheet of paper. US Letter 8.5 x 11 inches is based on something more arbitrary or t least lost in history.The method of determining it is defunct and it is just accepted.
The latter is used mostly in the U.S. Mexico and Canada but I believe the tow last countries also use A4.
A4 is seldom used in the U.S. that is the basis for determining what size to use.
You should ask yourself some questions:
What will you use it for? If finding the size for your standard letters, billing, correspondence, everyday-things like that you should go with what the audience can handle best: e.g. to archive your bills in their accounting office
If you rather want to send around your portfolio and show off your creativity, you might also use a different, non-standardized format to emphasize your creativity. Then you should ask yourself how much money you want to invest to get it printed and you should talk with the printer to find a compromise between a flashy format and the price you have to pay to get it produced.
Thanks guys, thanks for all your helpful answers.
For my part you are welcome, erinic.