Hi Cory,
It is calculated depending on the avarage result height and by the viewport count configuration: https://i.imgur.com/SbF6kYi.png
But of course you can fixate the maximum height of the area, when the scrollbar should always appear, use this custom CSS:
[html].asp_w .results {
max-height: 300px !important;
}[/html]
This way, if the results box reaches 300 pixels, it will automatically use the scrollbar.