We characterized the spectral response of CdTe sensors with different pixel sizes – namely 75, 150 and 300 μm – bonded to the latest generation IBEX single photon counting ASIC developed at DECTRIS, to detect monochromatic X-ray energy in the range 10–60 keV. We present a comparison of pulse height spectra recorded for several energies, showing the dependence on the pixel size of the non-trivial atomic fluorescence and charge sharing effects that affect the detector response. The extracted energy resolution, in terms of full width at half maximum or FWHM, ranges from 1.5 to 4 keV according to the pixel size and chip configuration. We devoted a careful analysis to the Quantum Efficiency and to the Spectral Efficiency — a newly-introduced measure that quantifies the impact of fluorescence and escape phenomena on the spectrum integrity in high-Z material based detectors. We then investigated the influence of the photon flux on the aforementioned quantities up to 180⋅106 cts/s/mm2 and 50⋅106 cts/s/mm2 for the 150 μm and 300 μm pixel case, respectively. Finally, we complemented the experimental data with analytical and with Monte Carlo simulations – taking into account the stochastic nature of atomic fluorescence – with an excellent agreement.