It would be great if rectangle (and polygon, circle, ellipse) layout text elements had fitting strategies like map labels. Legends already have some control for columns and text size. The idea is for all these elements to have additional fitting strategies like abbreviations and font width compression. This would help tremendously with ensuring layouts work for all pages of a map series with dynamic text of varying lengths.
This idea would help with fitting text in a single element, but a closely related idea of mine would help prevent overlap or clipping between multiple layout elements in a layout used for map series: