Make a "bounding box"
Add a cube to your scene, set its display type to wire to make it easy to see within. (and make it unrenderable yada yada). Scale it to fit all the parts of your model. Apply scale. Select all objects (or all topmost parents of) and make the cube their parent.
Now you can scale the cube. The dimensions of the cube will be the dimensions of all objects. Applying scale to the cube will filter down parent chain, applying scale to all children.
I use this technique when modelling something like a car with ortho plans. Make the cube at the start to the dimensions of car.