This example creates a new custom label named "Home Address" and then sets various properties for the label, including the page size.
Set myLabel = Application.MailingLabel _
.CustomLabels.Add(Name:="Home Address", DotMatrix:=False)
With myLabel
.Height = InchesToPoints(0.5)
.HorizontalPitch = InchesToPoints(2.06)
.NumberAcross = 4
.NumberDown = 20
.PageSize = wdCustomLabelLetter
.SideMargin = InchesToPoints(0.28)
.TopMargin = InchesToPoints(0.5)
.VerticalPitch = InchesToPoints(0.5)
.Width = InchesToPoints(1.75)
End With