Answer:
You mean "How tall WAS Thomas Becket?".
We are fortunate to have descriptions of St Thomas writen during his lifetime and soon after his murder. The monk Gervase of Canterbury wrote of him: "Thomas was of a goodly appearance, tall in stature, of a sharp intellect, sweet and pleasant in conversation, amiable in his manners, and of such keen powers of reasoning as to be able prudently to solve abstruse and difficult questions."
This only tells us that he was tall; a much more accurate idea of his height comes from surviving garments worn by St Thomas which are now in European cathedrals. A pale blue silk chasuble embroidered with gold, once worn by the Archbishop, measures 5 feet 4 inches high, giving an overall estimate of his height at around 6 feet (1.83 metres).
This makes him unusually tall for anyone in the 12th century - normal adult male height was around 5 feet 8 or 9 inches.