Occasionally a shape may be too small to fit all the text within the margins, and reducing the size of the text could make it difficult to read.

Decreasing the margins allows more text to fit on each line.

To change the margins, simply right click on the shape and then go to format shape.

The panel will default to the shape options, but to the right of that is a setting called text options.

Click on this and a new panel will appear.

There are three options in this panel, starting with text fill and outline text effects.

And finally, text box.

Click on text box, and at the bottom you will see the margins reduce these values to allow more text to fit on each line.