Most of the scale elements have relative size to one common dimension, which is called CellSize. If we set the relative height of the scale to be 0.9 that means that it will be calculated as 0.9 * CellSize. Thus all proportions of the scale elements will be preserved when the container size is changed.
If the property is not relative to the cell size it would be explicitly mentioned in the article regarding the particular feature.
The cell size depends on the scale type:
Linear Scale Cell Size
When using a linear scale, the cell size is equal to the height of the scale's container. Note that the cell size takes the entire height of the container. This means that if you have a LinearGauge with a height of 300 as a container, the cell size will be equal to 300.
Radial Scale Cell Size
When using a radial scale, the cell size is equal to the radius of the container. This means that if you have a RadialGauge with height/width of 300, the cell size will be 150.