I've never seen a way to make the default size different, but Debra Dalgleish
has some code to resize them whenever you want.

Maybe you can steal portions of that and modify it to size the way you like:
http://www.contextures.com/xlcomments03.html#Resize

Eddie wrote:

When inserting a comment to a particular cell, is it
possible to change the default size of the comment
window. I am trying to make the window bigger
automatically...without having to change it manually every
time.

--

Dave Peterson