When you insert a rectangle or a callout box using the drawing functions and activate the Points icon on the Drawing toolbar, you see a small frame at the upper left corner of the object. The frame indicates the amount by which the corners are rounded. When the frame is positioned at the top left corner, no rounding occurs. When the frame is positioned on the handle centered at the top of the object, the corners are rounded as much as possible. You adjust the degree of rounding by moving the frame between these two positions.
If you place the cursor on the box it changes to a hand symbol. You can now drag the box to change the amount of rounding. An outline shows a preview of the result.