Meagreness is the noun from meagre: the state of being too small, too thin, not enough. You can speak of the meagreness of a diet, a wage, a plot, or the evidence. It names the shortage itself, not the thing that is short. It is a formal, slightly heavy word; in speech people more often say “how little there was” or “how thin it was.” American spelling is meagerness. It always keeps that disapproving or pitying shade: this is less than a person or a task really needs.