When the LIST is displayed in a RadDropDownList, the datasource is usually a lookup table.  Therefore,  it is not practical to set the LIST attribute "DropdownWidth" to a fixed number of pixels, since items can be added to the lookup table where the text length is longer than any currently in the table.   If you text is too long for the LIST width, the text wraps and this is not a pleasant user experience.  

This control needs an autosize feature for the LIST, so it fits the longest text without causing wrapping.   This feature already exists on the RadComboBox with attributes DropDownAutoWidth="Enabled" and "NoWrap=true".   These should also be used I the RadDropDownList so the LISTS work identically.