I wouldn't know about that in a scientific way, though I would say it would be far to soon (for how many years have we had urban foxes, my guess would be around 40, in numbers) to change these animals phisically but from the 'old' days of hunting town foxes for years, most seemed of a smaller type, though the odd big 'un did turn up.
I belive diet and smaller territorys would play a part, foxes wouldn't have such a vast area to roam, then when in population, maybe not the best dog fox specimens would get to breed, which could produce smaller, weedier offspring?