If we assume that each list item is in its own paragraph...
If you want a border around the paragraphs then it should be done with a paragraph style. So the first step would be to create a paragraph style with a border and shading that you would like. Then you can run a macro like the following where 'Body Text' is the style you want for the multi-word paras.
Code:
Sub HighlightPhrases()
Dim aPar As Paragraph
ActiveDocument.Range.Style = "Normal"
For Each aPar In ActiveDocument.Paragraphs
If aPar.Range.Words.Count > 2 Then aPar.Style = "Body Text"
Next aPar
End Sub