Use attached project to reproduce. Workarond: radMultiColumnComboBox1.AutoSize = false; radMultiColumnComboBox1.MinimumSize = new Size(0, 22);
Declined, duplicate of: https://feedback.telerik.com/Project/154/Feedback/Details/166651-fix-radmulticolumncombobox-when-added-to-a-tablelayoutpanel-and-the-dock-prope