Replacing highly reflective envelope materials, retro-reflective materials (RRM) used as building coatings can help to mitigate the urban heat island (UHI) effect. Methods to measure retro-reflectance have been proposed by using different techniques. This paper presents a method to measure retro-reflectance by using the heat balance of RRM mounted on a plate in the outdoor environment. The accuracy of retro-reflectance evaluated by heat balance was corrected, with consideration of the heat losses from experimental reflection plate edges. We examined the influence of plate size on the measurement using plate size of 75 mm, 300 mm, 500 mm and 700 mm square. Results showed the 700 mm plate yields an accuracy of about 1% in measurement of retro-reflectance. The heat losses of plate sizes of 75 mm, 200 mm, 300 mm, 400 mm, 500 mm, 700 mm and 1000 mm square were also modeled by computational fluid dynamics (CFD) simulation. Results showed the proportion of heat losses to sides of plates evaluated by CFD simulation for the size of 700 mm square is only 1%, with little change for wind speed ranges from 0.2 to 0.8 m/s. It is consistent with the experiment results. Considering the difficulty of workability of a large plate, a plate size of 700 mm square was proposed as sufficiently accurate.