Paste the below code in the module and run it


Sub Comments_Style()
Dim MyComments As Comment
Dim LArea As Long
For Each MyComments In ActiveSheet.Comments
With MyComments
.Shape.AutoShapeType = msoShapeRoundedRectangle
.Shape.TextFrame.Characters.Font.Name = "Tahoma"
.Shape.TextFrame.Characters.Font.Size = 8
.Shape.TextFrame.Characters.Font.ColorIndex = 2
.Shape.Line.ForeColor.RGB = RGB(0, 0, 0)
.Shape.Line.BackColor.RGB = RGB(255, 255, 255)
.Shape.Fill.Visible = msoTrue
.Shape.Fill.ForeColor.RGB = RGB(58, 82, 184)
.Shape.Fill.OneColorGradient msoGradientDiagonalUp, 1, 0.23
End With
Next
End Sub

Change the shape style, font name, font size, font colour etc. as per your requirement.

To know all shape styles click on:
http://excelvbatipsforbeginners.blogspot.in/2012/08/all-shape-styles.html