Click on the 3D object file you want to change the dimensions of and the various properties of that will be shown in the design panel.
Look for the "Dimensions" section in the "Quick Access Option" tab and you can revise the size of the object by changing the values of width (W), height (H) and depth (D).
Alternatively, under the "Customize" tab you will find "Dimensions" section as well and select drop-down arrow next to it.