I did not manage to reproduce the issue you have described. When I set the DropDownWidth of the MultiColumnComboBoxElement to a specific value, the drop-down portion is resized according to that value. I am attaching a very simple project to demonstrate the correct behavior.
If your scenario is different than mine, please open a new support ticket and send me a sample project which shows the wrong behavior. This will allow me to address any potential issue further and to give you more accurate assistance.
As to the documentation, we have plans to further develop it for RadMultiColumnComboBox in the next couple of months. Your feedback will be of great help if you share with us what help topic would you like to see there.
I am looking forward to your response.
the Telerik team
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