What is the purpose of Auto Size with Overflow?
To cause the label fonts to auto-size and best fit in a region, and if they do not fit, allow overflowing into adjacent regions.
When is Auto Size with Overflow active?
When a map layer is selected and labels are turned on.
How do I access Auto Size with Overflow?
On the LABELS tab, in the Label Placement group, click Label Rules, and click Auto Size with Overflow.