I guess it depends on the country. In the U.S. it's 1.5 ounces.

http://en.wikipedia.org/wiki/Shot_glass#Sizes

More likely it has changed over time.

http://www.google.com/search?hl=en&source=hp&biw=135...

I'll admit, I'm old and my learning (through bartender's school, no less) reflects that. Most shot glasses will hold a jigger, but you don't fill it for a shot. Further down the page you linked, under "jigger," shows a tool we use(d). A "shot" was a drink, the "measure" was a jigger (1.5 oz) or a shot (1.0 oz). In programming terms, the word "shot" was overloaded.