Urban heat dome flow, which is also referred to as urban heat island circulation, is important for urban ventilation and pollutant transport between adjacent cities when the background wind is weak or absent. A "dome-shaped" profile can form at the upper boundary of the urban heat island circulation. The horizontal extent of the heat dome is an important parameter for estimating the size of the area it influences. This study reviews the existing data on the horizontal extent of the urban heat dome flow, as determined by using either field measurements or numerical simulations. A simple energy balance model is applied to obtain the maximum horizontal extent of a single heat dome over the urban area, which is found to be approximately 1.5 to 3.5 times the diameter of the city's urban area at night. A linearized model is also re-analysed to calculate the horizontal extent of the urban heat dome flow. This analysis supports the results from the energy balance model. During daytime, the horizontal extent of the urban heat dome flow is found to be about 2.0 to 3.3 times the urban area's diameter, as influenced by the convective turbulent plumes in the rural area.