Circumacenes and periacenes as zigzag-edged nanographenes differ only in that circumacenes have one extra benzene ring on each of the two armchair sides. Using first principles density functional theory, we show that this slight difference in the boundary shape dramatically affects the critical size at which the open-shell antiferromagnetic state supersedes the closed-shell nonmagnetic state as the ground state. We correlate this critical size to the decreasing HOMO-LUMO gap with the molecular size and discuss implications of the critical size for experimental syntheses of circumacenes and periacenes. (C) 2008 Elsevier B.V. All rights reserved.