You will have to attach a document with a sample of your text, because it works fine here as does a recorded version:
Code:
Sub Macro1()
Selection.Find.ClearFormatting
Selection.Find.Font.Size = 12
Selection.Find.Replacement.ClearFormatting
Selection.Find.Replacement.Font.Size = 8
With Selection.Find
.Text = ""
.Replacement.Text = ""
.Forward = True
.Wrap = wdFindAsk
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
End With
Selection.Find.Execute Replace:=wdReplaceAll
End Sub