Jokes aside, 2 meters outside it is probably a safe number due to several reasons, from the droplet travel distance and the evaporation factor (basically from the size of a droplet, it is not particularly complicated to calculate how far they can travel and how they can evaporate, partially or totally).
Consider that normal breathing runs at approximately 1m/s, coughing 10m/s, sneezing can go up to 50.
Having considered the above, a safe distance outside would probably be 2.5 meters (I can agree on 2 in certain scenarios) and I would consider 6 for indoor, however we cannot implement these solutions effectively, specially in crowded places like Hong Kong.
Let's look at the average 茶餐廳 / Cha Chaan Teng, if we had to implement a safe distance of 6 meters, we would most likely have 2xtwo seaters table: not doable.
So we are considering that the majority of droplets of.a certain size are usually evaporating before the 2 meters distance, and we have made a trade off with that, but that is absolutely not a safety distance, but let's say an OK trade off (that I would have not authorised for the whole end of February and March, if I were in the government hands).