Decks vary slightly in length, width, shape, concave etc, but the main thing most people are concerned about would be the WIDTH.

Deck widths typically range from 7.5" to 8.5", with increments of 0.125", which is about 3mm. The choice of width is purely a personal preference and is not tied to your shoe size or how many backflips you can do while giving a sealion high-fives.

If you are just starting out and do not know what width to get, I suggest going for a common width (as of 2010) of about 8", but really its not as big of a deal that most people make it out to be and you can skate just as well on any width.

As for LENGTH,
If you are like 2 years old and really small, you might want to consider a MINI skateboard. Also, congratulations! You are a genius if you really are 2 years old and reading this! For kids maybe under 10 years old who are less than 4 feet (1.2m) tall you might want a MINI skateboard as they are a couple of inches shorter and will be a better fit for kids with shorter legs.