To activate the feature, simply check "Display word count while typing" under Tools. When you click on the lower left corner where the numbers are, you can also view a panel with more information. It includes the document's character count (both with and without spaces), as well as the page count if you need to know if you have enough paper to print it out. Google says the feature is rolling out to all Google users and can be disabled in case you want to write without having to worry about how long the piece is getting.