I'm making my first complex scene and am trying to emulate the size of real world objects. I have a pack and a sword, which will be the main focal point of the piece. The kicker is, in my mind's eye, there's a huge mountain in the distance. I want to put it in "real world" scale so it sells the the distance and size when I set the camera f-stops. If I set my scale to miles or km, it gets really hard to find the pack and sword in the view port after I scale them to size. How do I go about using different such drastically different units of measurement in the same piece?
You can use outliner to select object and then numpad period "." key for centering it on the viewport.
In your preferences, check Auto Depth and Zoom To Mouse Position. This helps greatly when dealing with big distances. point your cursor over your sword before rotating, and you will see the difference.
Additionally, put your mountain on a different layer than the small objects. So, if you edit or animate them, switch the mountain layer off. Blender now can much better pan and rotate around your objects, because it can "think" and calculate in much smaller dimensions. And, of course, like the other answer said, use the numpad . to always center back to your selected object.
You could break it into two scenes: render the mountain landscape as a separate image, then bring that into the foreground scene as the background image. This will work well because it will show absolutely no perspective change as you move around your foreground scene, which will accurately reflect how far away it is.
It should also speed up rendering of the foreground scene.