The probability distribution P(k) of the sizes k of critical trees ( branching ratio m = 1) is well known to show a power law behavior k(-3/2). Such behavior corresponds to the mean-field approximation for many critical and self-organized critical phenomena. Here we show numerically and analytically that also supercritical trees (branching ration m > 1) are critical in that their size distribution obeys a power law k(-2). We mention some possible applications of these results.