To maintain the ratio of the height/width whenever you resize a shape:
- Select the shape
- Go to the Arrange tab in the format panel on the right hand side
- Select “Constrain Proportions”
While this is selected, resizing the shape will maintain the height/width ratio.
Note: you must set this for each individual shape - this behaviour is off by default. You can select multiple shapes and enable constrain proportions for all of them in one operation.
You can also hold down the shift key while resizing a shape to maintain the height/width ratio while resizing.