Hi,
The ComboBox API can be found on this page: https://www.htmlelements.com/docs/combobox-api/. There you will find dropDownWidth and dropDownHeight Javascript properties.
Example:

 <smart-combo-box drop-down-width="800" placeholder='please select whatever coffee you want to drink' selected-values=["Cappuccino"]>

Best regards,
Peter Stoev
Smart UI Team
https://www.htmlelements.com/