Perovskite colloidal nanocrystals have emerged as important new optical materials, with tuneable light emission across the visible spectrum, narrow linewidths for high colour purity, and quantum efficiencies approaching unity. These materials can be solution processed in large volumes at low cost making them promising for optoelectronic devices. The structure of nanocrystals influences the radiative and non-radiative recombination of carriers within them through trap states and Auger recombination. To optimise the emission properties it is vital to understand the relationship between the optical emission of individual nanocrystals and their structure, size, and composition. Here, we use nano-cathodoluminescence to relate the nanoscale optical emission of individual inorganic perovskite nanocrystals to their size. This approach reveals that larger nanocrystals exhibit brighter luminescence, indicating lower non-radiative losses compared to smaller nanocrystals. We also show nanoscale colour mixing with bright red and blue emission from individual CsPbI3 and CsPbCl3 nanocrystals respectively in mixed films. The optical and structural characterisation serve as a powerful approach to the study of colloidal semiconductor nanocrystals that improves the fundamental understanding of quantum structures leading to improved optoelectronic devices.