Customizing : Setting Units preferences
 
Setting Units preferences
You can set ruler options and the default resolutions for images that do not contain resolution information, such as scanned images. You can also set snapping for vector coordinates and enable document-aware tool sizing that lets you set the maximum tool size relative to the document size.
To set Units preferences
Edit tab 
Manage tab 
1 Choose File Preferences General Program Preferences.
2 From the left pane, choose Units.
3 Perform one or more of the following tasks, and click OK.
 
To
Do the following
Choose a unit of measurement for the rulers
In the Rulers group box, choose a unit of measurement option from the Display units drop-list.
Display rulers in black and white
In the Rulers group box, choose the Black on White option.
Display rulers in color
In the Rulers group box, choose the Toolbar colors option.
The ruler color matches the toolbar color.
Snap vector objects to the pixel’s center
In the Vector coordinates group box, mark the Snap to center of pixel while moving check box.
Set the default resolution for images that do not contain resolution information
In the Default Resolution group box, choose the unit of measure from the drop-list and enter a value in the control.
Adjust the maximum tool size relative to the document size.
In the Tool size area, mark the Enable relative tool size check box, and set a percentage to adjust the maximum tool size relative to the document.