setStyle("width","...")

var COMBO = new AW.UI.Combo;
COMBO.setId("COMBO");
COMBO.setControlText("Combo");
COMBO.setItemText(['Line 1','Long Line 2...','Line 3','Line 4']);
COMBO.setItemCount(4);
COMBO.setStyle("width","100%"); // !!!! It's not enought

// You have to set "PopupTemplate" as well, to set combo object width .
var template = COMBO.getPopupTemplate();
template.setStyle("width","100%");
COMBO.setPopupTemplate(template);


COMBO.refresh();
:)