This variation on Vivka's code avoids the selection object and hence it runs a lot faster.
Code:
Sub Italicize_If2()
Dim aRng As Range, iType As Integer
For iType = 1 To 2
Set aRng = ActiveDocument.StoryRanges(iType)
With aRng.Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = ""
.Replacement.Text = ""
.Font.Italic = True
.Forward = True
.Wrap = wdFindStop
.Format = True
.MatchCase = False
Do While .Execute
If aRng.Characters.Count < 30 Then aRng.Style = "myItalic"
aRng.Collapse wdCollapseEnd
Loop
End With
Next iType
lbl_exit:
Set aRng = Nothing
Exit Sub
End Sub