Size Objects
Click one of the Layout | Arrange | Size Both commands or the 
button to size one or more objects to a reference object's width, height, or both. At least two objects must be selected to use the Layout | Arrange | Size Both commands. The reference object's size does not change. All other objects are sized equally to the reference object. The reference object is the highest selected object in the Contents window, i.e. the front object in the plot window.
Width
Click Layout | Arrange | Size Both | Width or the 
button to set the width off all selected objects equal to the width of the reference object.
Height
Click Layout | Arrange | Size Both | Height or the 
button to set the height of all selected objects equal to the height of the reference object
Both
Click Layout | Arrange | Size Both to set both the width and height of all selected objects equal to the width and height of the reference object.
Changing the Reference Object
The quickest way to make sure you have the correct reference object is to select the object you wish to use as the reference object and click the Layout | Arrange | Bring to Front | Bring to Front command. The object is moved to the top of the Contents window and front of the plot window. It will be the reference object when the Size commands are used with any other objects. However, you may not wish to have your reference object at the front of the plot window. When this is the case, click and drag the desired reference object in the Contents window above the objects you wish to re-size. You can also use the Layout | Arrange | Bring to Front | Bring Forward command to move an object forward one place at a time.