Understanding light power values On Blender 2.80, lights (other than the sun) have various options for their strength. By default there is Power, measured in Watts, and a color option. If you enable "Use Nodes" you also get a generic Strenght value and another (seemingly ignored?) color option. 
I know that the Size option doesn't affect how much light is emitted, but just from where and also the shadow penumbra/softness.

But which value should I use when I set the power of my lights? For example, I have a simple scene with two area lights (30m in size), at the default power they were too weak to light the objects I have placed in the scene (it's a smoke simulation). I had to increase the Watts to 100000 to get the a good lightning. The lights weren't very close to the object, but not too far either. 

1000W with 1 in Strength is quite a faint light.
10W with 1000 in Strength is very strong instead.

Is Strength basically just a multiplier? Which means that the light emits 10.000W?

Which one should be used? Or how should I use both to get a "correct" (as in physically correct) result? Also does scale impact the amout of light? I usually set blender to centimeters instead of meters, because I use it with UE4 and UE4 uses centimeters as the unit.

Here is my blend file: https://we.tl/t-n4dS51e9AP
According to the documentation the strength value of the node should be in Watts, but this doesn't seem to be correct from experimentation.  The empirical behaviour of this seems to be as a multiplier.  So if you have a 1000W light and the strength set to 0.5 then you should have the effect of a 500W light.

Regarding color it is important that you set your light color to white if you intend to set the color of the node, this works in much the same way.

Concretely, it appears that nodes filter/amplify the light itself.  You can check this by setting you light power to 0W, no matter what you set the node values to, do you'll not produce any light.

I'm not sure what the intended behaviour is, but playing around with it suggests that a white emission shader of strength 1.0 will produce light exactly as it is configured in the light properties panel.

So the answer to your question is both can be used, it is up to you to decide if it is necessary to use nodes at all for this and just set the properties on the area light as you would expect.

Scale will matter as this defines the falloff of the light, however if you change the scale, blender will automatically compensate by changing the power level of the light.  So if you set everything up and then change the scale you shouldn't notice any difference unless you look at the power property of the light.