A couple of things. First, I agree with Andrew. Secondly, you are making use of more declared variables than you need. Thirdly, and this is just a matter of style and a recommendation, I would change variable naming convention.
Range is an object. Paragraph is an object. Range is a property of document object. Paragraphs is a property of the document object. Range is also a property of the paragraph object. So you can replace all this mash:
Set aRng = ActiveDocument.Range
aRng.End = ActiveDocument.Range.End
For Each aPara in aRng.Paragraps
with
For Each aPara in ActiveDocument.Paragraphs
Code:
Sub FontSize()
Dim oPar As Paragraph
For Each oPar In ActiveDocument.Paragraphs
If oPar.Range.Font.Size = wdUndefined Then oPar.Range.HighlightColorIndex = wdBrightGreen
Next oPar
End Sub
Since Range is a object, why not use oRng instead of aRng, oPar instead of aPar
strWhatever for strings, varWhatever for variants, bWhatever for boolean (e.g., bCheck instead of just Chect), lngWhatever for Longs etc.