From the File Menu, select View, and choose Properties window from the drop-down list or click on the Properties window icon in the toolbar. The Properties window will be displayed on the desktop.
Click on a Line control in the design window to display it's properties. The Properties window is also displayed when you drag and drop the Line Control onto the Format.
You can now set the properties for the selected Line control in the window as shown below.

Working with Field Controls | Setting Control Properties | Complete Property Listing | Line Class | Line Class Properties