The default is a log scale, where the vertical size of slave is based on the log of its range. This works best in many cases where there are large differences in slave size.
When showing slaves that are closer to the same size, then a linear scale is best. If there are two slaves and one has twice the range the size of the other, a linear scale would show the larger as twice as big as the smaller. If a slave would become very large it is drawn with a curved cut mark.
The constant size option shows all slaves as the same size regardless of the size of their range.
In the log and linear scales, the visual gaps between slaves also scales based on the size of the gap.
Ctrl-key+mouse wheel can be used to zoom in and out vertically.