Form: Draw Dimension Lines
Toolbar button:
Use the Draw menu > Draw Dimension Lines command to draw dimension lines at any location in a plan view. Dimension lines have arrowheads at each end, dimension text displaying the length of the line, and leader lines. Dimension lines cannot be drawn in a 3D view.
Click the Draw menu > Draw Dimension Lines command to access the floating Draw Dimension Lines form, which displays the Orientation (Parallel, Horizontal, Vertical) parameter that controls the orientation of the dimension line. If needed, modify the displayed data by selecting from the drop-down list before drawing the dimension line.
To place a dimension line in a plan view, left click on a grid intersection or at any other location to start the dimension line. Left click again at the point to dimension to. Drag the dimension line away from its original location to make the dimension line easier to read, and left click again.
To exit the draw dimension lines mode, click on the button 
or hit the Esc key to activate the select mode.