This is the smallest town in population and in surface area We would rather say this in English
Robert (my son's french dictionary left over from school) also defines 'en' as by so:
This is the smallest town BY population and BY area in this region
I agree. I'd say by area and by population
I agree; I was expecting to lose a heart for it too.
Tossing another agree in there!
...and another from me!
I would say, "This is the smallest town in or ("by", would also be okay, since this is what it is defined by) population and regional area"...but it marked it incorrect, oh, well...