What is the standard size of mahjong tile?

Answer 1

There is not such thing as a universal standard. Each region where a version of the game has developed has a size it tends to prefer, but usually a range of sizes is accepted. Tiles have gotten larger everywhere, with time, as a general rule.
Hong Kong, and Guandong in general, use particularly chunky (thick) tiles. The rest of China, lesss so. In general the preferred size in the continent is between 35 and 40 mm high, with Hong Kong (or Guandong) tiles being thicker than the rest.
Japan has traditionally preferred smaller tiles, which proportionally are also thick. Tiles 25 mm in height are the standard, but 28-30 mm ones are also frequent. Automatic machines have introduced larger tiles (up to 30, even 33 mm in height).
Taiwan's tiles have become increasingly thicker, in imitation of the Hong Kong sets, but theirs are not as large. There, 35 mm is the usual height.
In the United States, players who play the traditional American rules prefer tiles that run the slimmest. Around 30 to 33 mm in height, they are usually only 9 to 11 mm at most in thickness (sometimes less), and therefore require a rack to make them stand up, since they aren't very stable when standing up on their own.
It's important to remember that not everybody that plays in the U.S. uses the American ruleset, as Hong Kong, Official Chinese and Japanese are also popular.
Tom Sloper, in his website, Sloperama, adresses this topic, although the information is a bit dated, and as I said, tiles have grown in recent years (movies--where visibility is an issue--and automatic tables, being the main causes for this). However, check his comments (and pictures with measurements) at the bottom of the page called "Types of Sets", in the Mahjong section of his website, Sloperama.

Answer 2

8.5"*13