Experiments have been performed on turbulent, non-premixed, 25 kW, natural gas-oxygen flames and are based on measurements of the flame length by OH* emission observation. Various gas tube diameters have been used so that the ratio of flux mass by unit of surface ranges from 0.5 to 2.5. The oxy-fuel flame length, scaled by the characteristic dimension, d(f), introduced by Blake et al (1995) for air flames, yields asymptotically to the density weighted Froude number dependence of F-d(1/5). The coefficient of proportionality obtained for oxygen flames agrees well with the results for air combustion and shows that the analysis of Blake et al is pertinent to describe the height of high temperature flames. A comparison between free and confined flames shows that the confined configuration yields to 10 to 20% higher flame length compared with free flames. For identical exit conditions, the flame length can be modified by 20 to 30% by using a cylindrical or conical block.