Sorry if this is a stupid question, but: How do you manage extreme scales in Blender? I’m aware that I can scale the unit system, but that does not seem to affect the camera and there are also some other limits, such as the minimum particle size to which one might have to adhere. It’s also not clear how the camera settings would translate to real scaling. Eg.: do I halve the focal length if I want everything to look twice as large? And then there are also a bunch of other things that can come up, like simulation resolutions, huge clipping distance slowing Cycles down, etc.
do I halve the focal length if I want everything to look twice as large?
Treat the camera like a real camera, longer focal length has a narrower angle of view so the objects appear larger in the render.
And then there are also a bunch of other things that can come up, like simulation resolutions, huge clipping distance slowing Cycles down, etc.
For simulations work in metres (1 blender unit = 1 metre)
A large range in start/end clipping distances can cause visual anomalies so try not to use them. For such situations just fake the scene
Wouldn’t it make more sense to use… actual meters? Like… it’s right there, built in, so why use Blender units? 
As for the focal length, is there a way to calculate the exact size for scaling? (ie.: if the focal length is X and I scale the scene by a factor of F, how do I get the same results, what’s the relationship between X and F?)
Wouldn’t it make more sense to use… actual meters? Like… it’s right there, built in, so why use Blender units?
You can set units to metres.
1 blender unit can be whatever you want it to be, the length of a matchstick, the distance between Clackton and Timbuktu, a lightyear etc
As for the focal length, is there a way to calculate the exact size for scaling? (ie.: if the focal length is X and I scale the scene by a factor of F, how do I get the same results, what’s the relationship between X and F?)
Look for some online calculators such as http://www.vision-doctor.co.uk/optical-calculations/calculation-object-size.html