Is it possible to define the width and length of the comment in a cell by
Excel programming? Such a button in the Excel sheet, which add a comment in a
cell and this comment would have the length and width desired.

Experiment with the following:

'----------------------------------------------------------------------
Public Sub TestCommentSize()
Dim rngA1 As Range
Dim comA1 As Comment

Set rngA1 = ActiveSheet.Range("A1")

With rngA1
.ClearComments
.AddComment "This is a very big, very long comment to test sizing."
Set comA1 = .Comment
End With

With comA1.Shape
.Height = 100
.Width = 50
End With
End Sub

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Regards,
Bill Renaud