Diameter of a homogeneous cylinder can be determined with the spectrum of the rainbow pattern, formed by the scattering ray interference when the cylinder is illuminated by a laser. As for a coated cylinder, the intensity distribution and spectral characteristics of the twin primary rainbows under different cylinder sizes are investigated via theoretical simulation and experiments. The numerical simulation based on Debye theory is used to obtain the intensity distribution spectra and four characteristic frequency peaks are obtained, and how they are formed is also illustrated. The frequency peak F₃, with its origin similar to that of the ripple structure formed in a homogeneous cylinder, is then analyzed. The results show that F₃ increases with the outer diameter linearly and the slope is also slightly affected by the internal diameter and refractive index of the coated cylinder. However, there is no obvious relationship between the internal diameter and F₃. The experimental research is implemented by building a system to capture the rainbow pattern and its spectrum is analyzed. The relative error of outer diameter about 2% can be determined experimentally, when the internal diameter and the coated refractive index are fixed.