-[z]nor--window=nChanges the default scrolling window size to
nlines. The default is one screenful. Thezandwcommands can also be used to change the window size. The "z" may be omitted for compatibility with some versions ofmore. If the numbernis negative, it indicatesnlines less than the current screen size. For example, if the screen is 24 lines,-z-4sets the scrolling window to 20 lines. If the screen is resized to 40 lines, the scrolling window automatically changes to 36 lines.
Fonte: página man para less (Linux 2.6).
Nota: meus testes indicam que 0 é tratado como "negativo" neste contexto. O fragmento relevante deve dizer "se o número n não for positivo ...".