Private Sub TextBox_Custom_Size_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox_Custom_Size.KeyPress
        e.Handled = NumericOnly(e.KeyChar)
    End Sub
 
    Public Function NumericOnly(ByVal eChar As Char) As Boolean
         Dim chkStr As String = "0123456789,"
        If chkStr.IndexOf(eChar) > -1 OrElse eChar = vbBack Then
            If eChar = Chr(44) And TextBox1.Text.Contains(",") Then Return True
            Return False
        Else
            Return True
        End If
    End Function

Gracias, pero no

   If eChar = Chr(44) And TextBox_Custom_Size.Text.Contains(",") Then Return True