There are two ways to resize designs in the Designer. Firstly, using the Select tool, select the elements that you want to size with the Select & Move tool, then you can left click and drag on the sides of the dashed selection box. 

The second way is to once again select the design then simply type the size you want into the Width & Height fields. Please note this is defaulted to real world inches as the unit. This will automatically scale up your design proportionately. When you change the width, the height will also resize automatically to match. If you would like to squash or stretch your image, feel free to override this by unchecking the checkbox Proportional under the Width & Height fields.