The measure tool can be used to find the length of any straight line. The tool has many options for units of measure, both standard and metric. To open the measure tool, press ctrl+6 or scroll down the "tools" menu and click on it.
To use this tool, simply click the location you wish to start at and move the cursor to the desired endpoint and click again. This will measure the line drawn, and the length will be displayed in the box. To change the unit of measure, click on the arrow next to the word "miles" and select the unit you would like to display.
Once you are finished measuring, click the "Clear All" button in the box, and then close the window and you now can continue using Google Earth.