I just ran in to a situation where I could not increase the width of the Auto Complete extender with out extending the width of the associated control.Could not find workaround with the DLL it self and had to dig deep and found out a solution with the editing the following java scrip with in the source code in the file ‘AutoCompleteBehavior.js’ in Auto complete folder.
this._completionListElement.style.width = Math.max(1, elementBounds.width – 2) + ‘px’;
no matter if you manually set the width form the out side this line of code will set the width to bound element -2px so the solution was to either put a solid value to this
this._completionListElement.style.width = “400px”;
This is of couse if you are using it one size all the time
find a solution Here http://www.codeplex.com/AjaxControlToolkit/WorkItem/View.aspx?WorkItemId=13605