Good post hedo, those height ranges are accurate for romans at the time. However, we humans have NOT grown. Italians are still pretty short. Especially if you go back one generation.
At the time of the Romans, the northern peoples (Germanians, Gauls, scandinavians) were much bigger.
There is plenty of evidence of very big people from those times and earlier, whole races of people who are averaging 6'6" ... I realise some people think that isn't that big, but for an AVERAGE, it is frikkin' huge. Plus there must always have been the freaks who were exceptional, even the ones who were not suffering giantism.
People in general were about the same as now. People got bigger still around the year 1,000 ... then people started shrinking again after about 1350 when this mini ice age kicked in.
People were much smaller from say 1850 to 1920 in industrial places because the food basically stunk and they were poor, and pollution. London for example, you couldn't see the sky for all the smoke in the late 1800's.
From the point of evolution, we have hardly changed one tiny bit in 30,000 years, let alone in 2,000 years. The only thing that has changed is diet, lifestyle and knowledge.
Nobody ever said Achilles was a giant of a man. He was never represented as being a giant of a man, just athletic and very skilled.
Given all the above, and the fact that people at the time would have been doing quite a lot of hard labour their whole life, and not everyone was starving or lacking in good food (meat, unpasteurised dairy etc..) and also their love of the Olympics, it is reasonable to assume SOME people must have been rather big. Could they compare to todays' bodybuilders and all their science? maybe not ... could they compare to the bodybuilders of the 1950's? Quite possibly.