"C'est la plus petite ville en population et en surface de cette région."
I think that, in English, we would be more likely to translate 'surface' as 'area' rather than 'surface.'
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 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...