Ultrafine particles (with a small mean diameter) released from domestic coal combustion are an important parameter to consider in air pollution, as they affect air quality and human health. It has been suggested that poor combustion conditions release particles of different sizes enriched with health-damaging chemicals, such as polycyclic aromatic hydrocarbons. Furthermore, both smouldering and highly efficient combustion conditions release particles, which are often carcinogenic. Information on the particle size distribution (PSD) of char or soot emitted from fixed-bed domestic coal combustion is limited, with many studies reporting on wood combustion. This study investigated the influence of coal combustion phases (ignition, flaming, and coking) on the particle number concentration and size distribution of ultrafine particles. D-grade bituminous coal was crushed to a particle diameter (Ø) of 40–60 mm and combusted in a laboratory designed coal brazier (Imbaula) during experimental investigations of the particle size distribution normalised to the particle number concentration against the particle diameter. Experiments were carried out using the reduced smoke top-lit updraft method, colloquially known as the Basa njengo Magogo (BnM) method. The tests were carried out in a laboratory-controlled environment. Particulate matter was monitored using a NanoScan Scanning Mobility Particle Sizer (SMPS). Particles from the top-lit updraft (TLUD) showed an ultrafine geometric mean diameter centred at approximately 109 ± 18.4 nm for the ignition phase, 54.9 ± 5.9 nm for the pyrolysis/flaming phase, and 31.1 ± 5.1 nm for the coking phase. The particle mode diameter rapidly increased during the ignition phase (145 nm) and gradually decreased during the flaming phase (35 nm) and the coking phase (31 nm). This study shows that during smouldering combustion conditions (ignition), the particle diameter increases, whereas it decreases as the temperature increases. This information is essential for estimating particle deposition in the lungs and the associated health risks.