"20% smaller" is a pretty useless statement. There are three ways to interpret that statement that are all quite reasonable.
1) The Model S is 4.98m long and 1.96m wide, so reduce each by 20% and you get 3.98m long and 1.57m wide (smaller than the Leaf!)
2) You could reduce the area taken by the car by 20%, 4.98m x 1.96m is 9.76m2 - so reduce to 7.81m2 which could be 4.3m long and 1.8m wide (similar size to the Leaf)
3) There are 3 dimentions - so you should consider volume. The model S is 1.44m high, making it 14.05m3 in volume. Reduce that by 20% to 11.24m3 and you could have 1.4m high, 1.85m wide and 4.34m long (again similar in general size to the Leaf)
So my conclusion? Probably in the same ballpark as the leaf, but you could interpred "20% smaller" to mean all sorts of things.