There are lots of versions of Word and OS combos, so this may or may not work in your instance. But try this.

Right click on the text that fires the comment, and select “Edit Comment”.

That should bring up a box with draggable control points in the center of each side of the box. You know you are over the actual size control when the cursor changes to a double headed arrow.

I am on windows 7 (home basic) and Microsoft Office 2007.

In Word, go to review tab -> comments -> track changes -> change tracking options
Change tracking options
Please refer above image for that. Then, a pop up window will appear. In that, find preferred width in baloons as shown by following image.
options pop up window
Adjust the width as may please you.
Hope this helps.