The room dimensions are important for the frequency distribution of the normal modes of the room. The influence of the dimension ratio is analysed in box-shaped rooms with volumes between 25 m³ and 300 m³. Three different criteria have been applied to evaluate whether the frequency distribution is favourable: a smooth frequency response, the variance of the interval between modal frequencies, or the number of tones in the musical scale supported by at least one of the room modes. Clearly, a square room or a cubic room are unfavourable and should be avoided as music rooms. The results of the applied methods agree that there are three usable optimum dimension ratios, which have also been reported previously in the literature. The optimum ratios are (1:1.2:1.45), (1:1.4:1.89), and (1:1.48:2.12). However, it is also clear that nearly optimum dimension ratios are found within a certain range around each optimum. The analysis leads to a practical method for choosing favourable room dimensions in a music room.