What competition will you be doing?
IBCA does not certify judges. They use anyone off the street who is interested in judging and with that being said, RangerJ nailed it. 50 or Less teams will usually require 7 slices, generally about the width of a #2 pencil here in Louisiana IBCA comps. They will be parallel to the hinge of the box and the ends cannot be cutoff after cooking. If the slices are too long they have to be wrapped around the sides but still parallel to the hinge of the box. If more than 50 teams the head judges will normally increase the number of brisket slices, number of ribs and require two fully jointed 1/2 chickens.
Do a search for "Pictures of IBCA Brisket Turn In Boxes" and you will surely find some good examples.
You can find the IBCA rules here: