We have studied sidebranching induced by fluctuations in dendritic growth by means of a phase-field model. We have considered a region where the linear theories are not valid and we have computed the contour length and the area of the dendrite at different distances from the tip. The dependence of the ratio of both magnitudes with the undercooling shows a behaviour in agreement with previous experiments. The derived scaling relation implies that dendrites are self-similar in the considered region.