Ethylene dehydroaromatisation (EDA) was investigated at 700 °C under 1 bar of ethylene (5 mol% in N2) over a micro-(M) and a nano-sized (N) H-ZSM-5. On the M zeolite an induction period followed by deactivation was observed, which could be related to the presence of long diffusion path lengths in this sample, leading to mass transfer resistance. During the induction step, the aromatics yield increases, despite a significant loss of the acid site concentration as a result of coking. This induction period corresponds to the formation of an active hydrocarbon pool (HCP) composed of units of 2 to 5 aromatic rings with a molecular weight ranging from 130 to 220 g mol−1 (light coke). A kinetic study revealed that the developing HCP species is two times more active than Brønsted acid sites in the fresh zeolite. Diffusion limitations yet impact the product desorption by promoting coke growth and, therefore the deactivation of the HCP and hence of the catalyst. From MA-LDI/LDI-TOF MS (Matrix Assisted Laser Desorption Ionization—Time of Flight Mass Spectroscopy) characterisation was deduced that even after complete catalyst deactivation, the as-deposited coke continues growing at the external surface of the zeolite by condensation reactions, thus leading to heavy coke composed of more than 100 carbon atoms and a molar mass exceeding 1300 g mol−1. Unlike the micro-sized zeolite, the nano-scaled zeolite features a short diffusion path length and promotes fast formation of the active HCP. As a result, higher activity and selectivity into benzene were observed, whilst catalyst deactivation was significantly mitigated.