During a fire in a tunnel, the temperature is a crucial parameter for the occupants of the tunnel, for the ability of the rescue service to approach the fire, and for the tunnel structure. Therefore, it is of importance to be able to derive information on temperature development and the maximum temperature for different fire scenarios and tunnels with different heights and widths. Fire tests in a model-scale tunnel have been performed to study the effect of the width and height of a tunnel on the gas temperature inside a tunnel during a fire. The model-scale tunnel was 10 m long. The widths used were 0.3 m, 0.45 m, and 0.6 m and the heights 0.25 m and 0.4 m. Fire tests with heptane and wood cribs, respectively, were performed. There was an effect on the gas temperature of the tunnel dimension, generally decreasing temperature with increasing dimensions, even if the effect varied at different positions in the tunnel and the effect in some positions was relatively small. Also, the effect in the vicinity of the fire was opposite to the effect further downstream of the fire.