To get back on topic, the Model S seems to be 1964mm with mirrors folded and 2189mm with mirrors extended (from the Tesla specs page).
For the Hong Kong market (where I live), that is the absolute maximum I could accept. The Model S barely fits in parking spaces as it is - if the Model X is any wider, it is just not going to sell.
On a related note (sliding back off topic), Tesla also need to take care with the height. Model S height is 1435mm. Anything over 1800mm just won't fit in most car parks here.
Bottom line is that I know Americans are fond of large cars, but if Tesla want to sell cars outside America they need to curb their enthusiasm.