What is the evidence that the Buddha was a giant?

Although this is because of a mistranslation of the units. There were 2 units of measurements in Sri Lanka back when the Buddha was alive. Both are suffixed ‘Riyana’ unfortunately.

• The correct ‘Riyana’ is the ‘Wadu-Riyana’. This is approx 4 inches. 
• Wrongly assumed height = 18 Riyana = 18 x 12 inches = 18 feet
• Correct height = 18 Wadu-Riyana = 18 x 4 inches = 6 feet
• That makes the actual height of Gothama Buddha to be about 6 feet tall.

Further , The learned Brahmins like Ambatta Manavakaya, who came to meet the Buddha, always searched for things like whether he had the characteristics of a great man, whether he belonged to the Arya tribe by tribe, but these tribes who were in the forest areas of the state searched for Buddha’s spiritual skills. – Hemavata Sutta

The hero so lean, with antelope calves,not greedy, eating little, the sage meditating alone in the forest. come now, let us see Gotama.- Hemavata Sutta