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Taking the question Who was the tallest Roman ever found? to mean archaeological remains (the most reliable evidence we are likely to find), the current leading 'candidate' is probably a skeleton labelled T.30 which was
...found during archaeological excavations in the territory of Fidenae, an administrative centre of the Roman territorial organization, situated along the Via Salaria about 7 km north of Rome (Italy). The individual was a young male, dated back to the Imperial Age (3rd century AD), presenting a very tall but normally proportioned stature, estimated around 202 cm [6ft 7.5in]. The long bones showed incomplete epiphyseal union, therefore the stature would probably have been taller, if he had lived longer.
Source: S. Minozzi, W. Pantano, P. Catalano & F. di Gennaro βThe Roman Giantβ: Overgrowth Syndrome in Skeletal Remains from the Imperial Age. In 'International Journal of Osteoarchaeology 23:1-11' (May 2013)
This may not seem much by modern standards but considering that a male Roman in the Imperial Age is estimated to have averaged 167cm (just under 5ft 6in), the gentleman who was skeleton T.30 would have been unusually tall. The skeleton, which the authors say "is characteristic of a form of gigantism", was originally found in 1991 in a cemetery probably connected to a farming community.
A total of 31 tombs were excavated and the skeletal remains of 28 individuals were recovered. A tomb (T.30) longer than the others was found (2.6 m) and the skeletal remains appeared to be those of a very tall individual (Pantano et al., 2011). Tomb 30 belongs to a group of simple fossa graves (originally covered with pottery roof tiles) that seems to have been prepared and used during the first half of the 3rd century. The individual was buried in supine posture with the hands placed under his pelvis, with no funerary goods. The skeleton was complete and in good condition, but the skull was fragmented.
Source: Minozzi et al (2013)
Text & images source: S. Minozzi, W. Pantano, F. di Gennaro & G. Fornaciari, 'Pituitary Disease from the Past: A Rare Case of Gigantism in Skeletal Remains from the Roman Imperial Age'. In 'The Journal of Clinical Endocrinology and Metabolism' (October 2012)
We cannot be sure that the skeleton was that of a Roman citizen and, unfortunately, the article does not go into details of the likely birthplace of this individual. Other than the above,
we have no reports about their [giant individuals] presence in the Roman world during the Imperial Age. The only exception was the Emperor Maximinus the Thrax (235-238 AD), described by literary sources as a human mountain, and according to the image on his coinage had an acromegalic head; therefore he may well have been a giant (Roberts, 1978).
One of the literary sources for this is the Historia Augusta which, unfortunately, is not very reliable so we cannot be sure of his true height, though there seems to be little doubt that he was a very large individual (and quite possibly taller than the 'owner' of skeleton T.30). Another source, Herodian, also mentions that Maximinus Thrax was a very man but gives nothing more specific than the following:
...he enlisted in a local auxiliary cohort because of his huge size and great strength, and by luck became the emperor of the Romans.
Source: Herodian 7.1.2
The emperor's appearance was frightening and his body was huge; not easily would any of the skilled Greek athletes or the best-trained warriors among the barbarians prove his equal.
Source: Herodian 7.1.12
Two other primary sources, Zosimus and Zonaras, do not mention his size (though neither were contemporaries of Maximinus Thrax.
(all emphasis is mine)