Creating a line dimension is just like using a
measuring tape. It’s a quick way to calculate the distance between two
points.
To make a line dimension, select the add line dimension tool, located
in the editor toolbar at the top of the
screen. Click once in the work area to set
the location of the first point. As you move your cursor, you’ll see a
real-time indication of the distance between the first point and your
cursor. Click again to set the location of the second point. When you
set the second point, you’ll see the line remains with the distance indicated
on the line.
Note: If the second point that you pick is within a few degrees of true vertical or horizontal, the dimension line will automatically snap to true vertical or horizontal. If your intention is to have a dimension line as a slightly-off angle, edit one end after placing it.
Note: The "Snap Walls 90°" command affects line dimensions the same way as it does wall. Toggle this setting to control whether or not line dimensions will snap to 45° increments.
Click HERE for other dimension related commands.