'参照設定に System.Design.dll が必要。
Imports System.Windows.Forms.Design
Imports System.ComponentModel

<Designer(GetType(MyTextBoxDesigner))> _
Public Class MyTextBox
    Inherits TextBox

    Protected Overrides ReadOnly Property DefaultSize() As System.Drawing.Size
        Get
            Return New Size(120, 19)
        End Get
    End Property

    Public Overloads Property Size() As Size
        Get
            Return MyBase.Size
        End Get
        Set(ByVal value As Size)
        End Set
    End Property

End Class

Friend Class MyTextBoxDesigner
    Inherits ControlDesigner

    Public Overrides ReadOnly Property SelectionRules() As System.Windows.Forms.Design.SelectionRules
        Get
            Return MyBase.SelectionRules And Not SelectionRules.AllSizeable
        End Get
    End Property

End Class