To reproduce: please refer to the attached sample screenshots and project.

Note: this issue is relevant for RadDropDownList as well.

Workaround:             
            this.radDropDownList2.DropDownListElement.TextBox.TextBoxItem.Multiline = true;
            this.radDropDownList2.DropDownListElement.TextBox.TextBoxItem.MinSize  = new Size(30, 15);