When you set the DropDownHeight property to a big number that exceeds the screen's height, the applied values is SystemInformation.WorkingArea.Height - 100 . However, it should be only SystemInformation.WorkingArea.Height. These 100 pixels shouldn't be lost.

NOTE: the same behavior is observed with DropDownWidth as well.

Workaround:

Public Class CustomMultiColumn
Inherits RadMultiColumnComboBox

    Protected Overrides Function CreateMultiColumnComboBoxElement() As RadMultiColumnComboBoxElement
        Return New CustomRadMultiColumnComboBoxElement()
    End Function

    Public Overrides Property ThemeClassName As String
        Get
            Return GetType(RadMultiColumnComboBox).FullName
        End Get
        Set(value As String)
            MyBase.ThemeClassName = value
        End Set
    End Property
End Class

Public Class CustomRadMultiColumnComboBoxElement
Inherits RadMultiColumnComboBoxElement
    Protected Overrides Function GetPopupSize(popup As RadPopupControlBase, measure As Boolean) As Drawing.Size
        Dim s As Size = MyBase.GetPopupSize(popup, measure)
        Return New Size(s.Width, Math.Min(SystemInformation.WorkingArea.Height, Me.DropDownHeight))
    End Function

    Protected Overrides ReadOnly Property ThemeEffectiveType As Type
        Get
            Return GetType(RadMultiColumnComboBoxElement)
        End Get
    End Property
End Class