Jeff,

I don't know how the default size is determined. I would creating a
new workbook and insert a comment. Then, with that cell selected, Alt
F11 to go to VBE. Then, on the Menubar, View Immediate Window. Enter
the following statements to get the width and height.

?activecell.Comment.Shape.width
?activecell.Comment.Shape.height

Mine are 108 and 55.5, respectively.

Sub ChangeCommentSize()
Dim cmt As Comment
For Each cmt In ActiveWorkbook.ActiveSheet.Comments
With cmt
.Shape.Width = 108
.Shape.Height = 55.5
End With
Next cmt
End Sub

Tested using Excel 97SR2 on Windows 98SE,

HTH
Paul
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Be advised to back up your WorkBook before attempting to make changes.
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Sorry I don't want to change the default size, I want to
change them BACK to the default size.