We, frankly, don't know a lot about Þorkell hávi - he was likely a historical figures, as the so-called "Anglo-Saxon Chronicle" refers to him repeatedly in the years 1009-1017. He seems to have been a pretty well-respected leader, at least in charge of 45 ships of men who serves as mercenaries for Æthelred unræd in 1013-1016. If the account of his childhood is to be believed (spoilers: it isn't), he was the son of Strút-Haraldr, the earl of Sjælland, so he was of noble descent. He became a leader among the legendary Jomsvikings, a group whose historicity is still debated. The saga about them dates from 13th century Iceland, despite being set in 10th century Denmark, and so many scholars have argued that it should be read as historical fiction. So, all we know is that by the time he shows up in more typically reliable sources, he's been given that epithet.

Even this unreliable Jómsvikinga saga, the only account of his childhood we have, gives only the most straightforward explanation of his epithet. To quote from the saga - "Þorkell ... var manna hæstr ok sterkr ok forvitri, ok hvárrtveggi þeira"

"Thorkell... was the tallest of men, strong and wise, as each of the two of them [i.e. Thorkell and his brother] were."

This is, regrettably, all we have. Regular measurements of height are not a thing in medieval Icelandic literature, and his skeleton has never been found to actually measure. But, it's safe to say he was substantially above average for people in his time.