You can choose to allow the characters of labels to be spread. Spreading the characters within a label can allow the Maplex Label Engine to make text fit linear features better without overlapping other labels. When you choose to allow the characters of labels to be spread, you can set a maximum character spacing. The character width of the labels' text symbol is used as the minimum spacing.
This parameter is available for the Centered Curved line placement style. When labels are being placed with the Street Placement option, the Spread characters parameter can also be applied with the Offset Curved, Centered Straight, and Offset Straight line placement styles.
If you set the maximum value to 0, the labels will be stretched to fit the length of the feature in the map extent.
The Spread characters option works within a given spread range. The lower limit is set on the Text Symbol Properties dialog box. This is the default character spacing if you do not choose to spread the characters in your labels. The Maplex Label Engine allows you to specify the maximum value of the character spacing range for certain line-labeling styles.
The image below shows the street labels set up with a spread characters maximum distance of 0. You can see that Kennedy Drive is stretched along the length of the street, as is 47th Avenue.