Actually it does matter. If you open a scene I did using CM, and you use inches, it will open and you'll get the values in inches, which will be different. 2000 inches is 5000 cm (actually slightly more, but just about anyway). So that's quite a difference in units. Especially when dealing with something like this 2000 or 5000 is quite a difference. If you didn't have to care for the way the angle of the light looked, and we were just talking about scale, then it wouldn't matter, but in this particular case it does, so kind of interesting to know whether you use metric or "Ancient British Stuff That Even Britain has Decided to Get Rid of", which I like to call it as I've never heard anything about a name for it.