I've seen this in the forum before:
Column Widths of Dropdown List Table Mode
...but perhaps I did something wrong and didn't realize it when I experimented with this a couple of mornings ago because, at the time, I couldn't get the original scrollbar to accept my size adjustments. This is much simpler, and it gives the desired result:
from java.awt import Dimension
newWidth = 50
dropdownComponent = event.source.parent.getComponent('Dropdown')
dropdown = dropdownComponent.UI.getAccessibleChild(dropdownComponent, 0)
scrollPane = dropdown.getComponent(0)
scrollbar = scrollPane.verticalScrollBar
scrollbar.setPreferredSize(Dimension(newWidth, scrollbar.height))
Result:
