While I don't think you can scale an assembly automatically, you can scale parts without having to edit dimensions individually. In Wildfire 3, in the Edit Menu, there is a Scale Model option.
Open your assembly, show your Model Tree, expand all subassemblies (filtering out features may make this easier), select your first part and then "activate" it. Select Edit/Scale Model and enter your the scale value. It should ask you to select yes or no; yes will complete the action and no will revert to the old value. Activate the next part and "scale" again. Once you've completed all parts in the assembly, choose your top assembly and "activate" it. Finish by regenerating. Depending on your assembly relationships, there may be some cleanup to do.
Large assemblies can prove to be quite time consuming, so if anyone else has a better method, I'd like to know.