What version Publisher?
2002-03, select custom in page setup, type 2.5 width, 2.5 height, click change
copies per sheet, use these measurements:
Side>.25
Top>.5
Horizontal Gap>.25
Vertical Gap>zero
Publisher will default to 3 columns of 4.
In earlier versions of Publisher you will have the option to change the margins
and gaps when you are ready to print.
If you are using round labels, it will be helpful to create a 2.5 circle on the
publication. It will help keeping your creation centered. When you are finished
doing your design, delete the circle.
Round labels are the most difficult to print...