Hello Stephan,

 Thank you for your post. I have been looking into it. After some research I have made for you, I can suggest to use the xamMultiColumnComboEditor as EditorTemaplate for the xamGrid. The editor has a property SelectedItemsResetButtonVisibility, which can be set to false and the mentioned button would not be visible. The XamMultiColumnComboEditor exposes a DropDownOpend event, which can be used for setting Width for the dropdown itself. Here is an example on how the width property can be set in the DropDownOpened event:

          Grid grid = VisualTreeHelper.GetChild(

                    sender as Infragistics.Controls.Editors.XamMultiColumnComboEditor, 0) as Grid;

                Popup popup = ((grid.Children[0] as Border).Child as Grid).Children[0] as Popup;

                 ((popup.Child as Grid).Children[0] as Border).Width = 500;

 Please do not hesitate to let me know if you have any further question on the matter.