Access this via Edit > Preferences > Comment.
You can specify the appearance of comments in your PDF documents. In the TextBox Default Font area, select a font name, size and an underline style from the drop-down lists. Choose a font color and specify combinations of other text attributes such as Bold, Italic, Superscript, Subscript and Strikethrough.
In the Text Box Default Align area, choose Left, Center, Right or Justified alignment for your text in comment boxes.
Click Reset Comment Defaults to restore delivery-time settings.
See also: Comments Panel.