The "Default Scale" of your instances.
If not toggled, the default scale will be of (1,1,1).

The "Space" method will enable you to define if your instances should scale with their local surface scale, or stick to the (global) world-space. If the default scale feature is disabled scatter will automatically pick the same space of your distribution method.

Reminder:
Don't forget that in blender, it's important to apply the scale of an object, when you consider that this object is in its default scale.

The "Random Scale" feature will randomize the scale of your instances, uniformly or per vector elements.

The "Shrink" & "Grow" features will multiply your scale by the given vectorial factors.

These two settings are meant to be used with a feature mask, to easily and precisely control your instances scaling with the help of weight painting for example.

The "Scale Fading" goal is to intensify your scenery perspective by emphasizing your instances scales.

Note that this feature will rely on the active camera & its dependencies. This effect is not suited for all use cases.

In order to hide the repetition of your instances, the "Random Mirror" feature will come handy!
It will randomly multiply your scale vector by -1 on chosen X/Y/Z axis.

Of course, try to avoid using this feature with instances containing writings of any sort of logo.

It is quite common to stumble on instances that are way smaller than reality in your scatter-system(s).
That's where "Overall Minimal Scale" comes in handy, this feature will remove or adjust scales of instances below the given threshold.