I googled your issue and found this:
http://www.dummies.com/how-to/content/how-to-work-with-worksheet-comments-in-excel-2011-.html
I will add this note - other post I found said that you can change the size but it will not "stick" and it reverts back. Also nothing on how to change the default.
Maybe they fixed this in 2016? I am still using 2011 so I have no way of checking it out.
Lisa