To clarify what Clockwatcher means for anyone who hasn’t done this before, here’s a step by step guide:
1) Drag a new mesh like a Cube into your scene, then rename it SCALER (select the actor in the world outliner and press F2).
2) Press CTRL + A to select your entire level (or just select the actors you want to scaler) and then hold CTRL and Left click on your SCALER actor to de-select it.
3) In the World Outliner, right-click on any of the selected actors and choose Attach To and then pick your SCALER mesh in the list (Alternatively you can drag all of the selected objects in the world outliner onto the SCALER mesh to parent/attach.)and then deselect everything. In the World Outliner, you will see that your SCALER mesh is now at the top of the list with a grey arrow (black when selected) to the left of its name and everything else is below it and slightly indented to the right letting you know that they’re all children of the SCALER mesh.
4) If you know the exact amount you want to scale everything, select your SCALER mesh and hit the lock icon to scale uniformly then type the amount in one of the axes. For example 4.0 for 4X larger or 0.25 for 4x smaller. You can also use the transform gizmo (Press R or hit Spacebar till you get to scale) to scale by eye if you’re not sure of the exact amount.
5) Once everything is to the scale you want, just delete the SCALER mesh and it will unparent everything else.