On the Design tab, in the Page Setup group, click the Page Setup Dialog Box Launcher.
Click the Page Properties tab. In the Measurement Units list, select the units you want, and then click OK.
Change the spacing of ruler subdivisions
To change the spacing of ruler subdivisions, on the View tab, in the Show group, click the Ruler & Grid Dialog Box Launcher. In the Subdivisions lists, select the spacing you want.
What about pixels as measurement units?
For some types of drawings, you may want to change the measurement units to pixels. However, a pixel isn't a unit of measurement. A pixel is just a dot on a screen and the size of the dot varies for different screens. To simulate pixels, set the measurement units to points.