Thank you for writing back.
The drop-down popup that contains the items and the autocomplete drop-down popup are different objects and when you set a property to the first popup, this setting is not applied automatically to the second one. So, if you want to change the SizingMode of the autocomplete popup, you should apply this setting to the DropDownList of the AutoCompleteSuggest object:
radDropDownList1.DropDownListElement.AutoCompleteSuggest.DropDownList.DropDownSizingMode = Telerik.WinControls.UI.SizingMode.None;
This will prevent the users from resizing the AutoCompleteSuggest popup.
We would kindly ask you to use just one support channel to ask one specific question. Posting the same questions more than once may slow down our response time, because we need to review and address two or more tickets instead of one. Thank you for your understanding.
Do not hesitate to contact us if you have other questions.
All the best,
the Telerik team