  The fluorescence technique has been successfully used to detect ultrahigh
energy cosmic rays by indirect measurements. The underlying idea is that the
number of charged particles in the atmospheric shower, i.e, its longitudinal
profile, can be extracted from the amount of emitted nitrogen fluorescence
light. However the influence of shower fluctuations and the very possible
presence of different nuclear species in the primary cosmic ray spectrum make
the estimate of the shower energy from the fluorescence data analysis a
difficult task. We investigate the potential of shower size at maximum depth as
estimator of shower energy. The detection of the fluorescence light is
simulated in detail and the reconstruction biases are carefully analyzed. We
extend our calculations to both Auger and EUSO experiments. This kind of
approach is of particular interest for showers that are not fully contained
inside the field of view of the detector.
