We printed a
bunch of 8.5" by 11" sheets and taped them together to make a full-size target.
We also made a small version (just 8.5" by 11") on cardboard, which was great because we could tape it onto a PVC pipe and have people wave it around for the robot to track. Also, that way the person holding it could be safely out of the way when the robot decided it had found the target and it was time to fire
.