This clumsy syntax incorporates the default namespace:
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim path As String = "C:\test\157.xml"
Dim n As XmlNode
Dim xm As New XmlDocument
Dim nsmgr As New XmlNamespaceManager(xm.NameTable)
nsmgr.AddNamespace("q", "http://schemas.microsoft.com/deepzoom/2009")
xm.Load(path)
n = xm.SelectSingleNode("/q:Collection/q:Items/q:I/q:Size", nsmgr)
Debug.Print("width = " & n.Attributes("Width").Value & " height = " & n.Attributes("Height").Value)
End Sub