I don't know a way of changing that default.

Maybe you could just copy and existing comment and edit|paste special|comments.

Or maybe you could use a macro to add the comment:

Option Explicit
Sub testme()
   With ActiveCell
       If .Comment Is Nothing Then
           'do nothing
       Else
           .ClearComments
       End If
       
       .AddComment Text:=Application.UserName & ":" & vbLf
       
       .Comment.Shape.Placement = xlMoveAndSize
   End With
   SendKeys "%ie~"

End Sub

Or don't worry about that setting when you're adding the comments--just come
back and toggle that setting for all the comments:

Option Explicit
Sub testme2()
   Dim cmt As Comment
   For Each cmt In ActiveSheet.Comments
       cmt.Shape.Placement = xlMoveAndSize
   Next cmt
End Sub

But those comments still seem to have a mind of their own.

Debra Dalgleish has sample code for working with comments
(resizing/repositioning):
http://contextures.com/xlcomments03.html