With the tail still selected, click the "Select" part of
the menu bar and find the "Modify" area. From the list select "Contract" which will
bring up the box you see. You can contract it based on how thick you want the balloon
borders. I tend to have pretty thick borders for my balloons so I contract it by "2."
I then select the white color and using the Paint Bucket Tool and fill in the selected
area. This will create a border and it should match the same thickness as your balloon.