Who were the "Nefilim"?
Rashi: The Nefilim were giants, descendents of Shamchaza'i and Aza'el, who fell from heaven in the days of Enosh. 1
Why did the Spies repeat "Min ha'Nefilim"?
Ramban, Rashbam and Targum Yonasan: To inform the people that they were from the original Nefilim mentioned in Bereishis 6:4 (who were synonymous with the three sons of Anak 1 and who were renowned for their awesomeness 2 ).
Why did the Cana'anim compare them to grasshoppers?
Rashbam: Because a tiny person appears in the eyes of someone who is very tall like a grasshopper (which is the epitome of a very small species). 1
How did the Spies know how they appeared in the eyes of the Cana'anim.
Rashi: Because they heard them say to one another 'We saw ants 1 - some have the text locusts - in the vineyards that resemble men!'
See Ba'al ha'Turim. The commentaries also explain the Pasuk to mean that when a person belittles himself then that is how others see him.
Seeing as they were the Nefilim, why did the Spies mention the "b'nei Anak"?