Hello algunderson,

I assume you are asking about the dropdown list container, because the width of the actual dropdown control by default takes the whole width of the cell. If this is the case, you could get the width of the DropDownProvider and use it to set width of the items list container, for example in DropDownOpening event:

 function DropDownOpeningHandler(sender, eventArgs) {
    var width = sender.get_element().style.width;
    document.getElementsByClassName("igdd_DropDownListContainer")[0].style.width = width;
}

Let me know if this helps.