@digdoug Jonathan Williamson uses Blender Units in this video (which are no longer available in 2.8), but that is the same as Metric. The only difference is that for instance the Default Cube is 2 by 2 by 2 in Blender Units and 2m by 2m by 2m in Metric.
So 0.338 in the video corresponds to 0.338 meters. In your image you also have meters, so that is not the difference. As your scale is also 1, 1, 1 the only thing I can think of is that your Background Image is smaller than his.
Anyway, like @blanchsb already said: don’t worry about the exact numbers, just make them more or less ‘square’.
