Try changing AutoWidthOption in the properties for the box.
Manual: It will look at the number of pixels entered in the Col1ManualWidth property for how wide to make the dropdown.
ControlWidth: The dropdown will be as wide as the box.
Auto: Epicor will guess how wide to make the dropdown to make the contents fully visible. It's not always correct. If the dropdown for a box spans the width of the screen, change to ControlWidth or Manual to fix it.