Yo lo hago con ésta función:
Public Shared Sub Maximizar()
Dim strScript As String
strScript = "top.window.moveTo(0,0);" & vbNewLine
strScript &= "if (document.all){" & vbNewLine
strScript &= " top.window.resizeTo(screen.availWidth,screen.availHeight);" & vbNewLine
strScript &= "}" & vbNewLine
strScript &= "else if (document.layers || document.getElementById) {" & vbNewLine
strScript &= " if (top.window.outerHeight < screen.availHeight||top.window.outerWidth < screen.availWidth){" & vbNewLine
strScript &= " top.window.outerHeight = screen.availHeight;" & vbNewLine
strScript &= " top.window.outerWidth = screen.availWidth;" & vbNewLine
strScript &= " }" & vbNewLine
strScript &= "}" & vbNewLine
Dim exPage As Page = CType(HttpContext.Current.Handler, Page)
exPage.ClientScript.RegisterStartupScript(exPage.GetType, "maximizar", strScript, True)
End Sub