Sub test()
Dim S As String
Dim l As Long
Dim i As Long
Dim j As Long
Const strSize As Long = 20
S = Range("A1").Comment.Text
l = Len(S)
For i = (l \ strSize) * strSize To 1 Step strSize * (-1)
S = Left(S, i) & vbLf & Right(S, l - i + j)
j = j + 1
Next
Range("A1").Comment.Text Text:=S