What is the meaning of "Ki She'al Na le'Yamim Rishonim ... u'le'Miktzei ha'Shamayim...'?
Rashi #1: The Pasuk means "inquire now about the early days ... and ask the creatures from one end of the heaven to the other".
Rashi #2 (citing Chagigah 12a): "U'l'Miktzei ha'Shamayim ad Ketzei ha'Shamayim" follows from "asher Bara Elokim Adam al ha'Aretz", and it teaches us that when Adam was created, his height was from the earth to the Heaven and from one end of the Heaven to the other. 1
Targum Yonasan: It means that one should inquire (not about the days, but) from the generations who lived in the early days from the day that Hashem created Adam on the earth and from one end of the Heaven to the other. 2
Why does Moshe insert these Pesukim here?
Ramban: To teach us why Hashem will not forgive Yisrael, even short-term, for the sin of Avodah Zarah or of the manufacture of images - because He did for them what He did for no other nation, to allow them to hear His Voice from the midst of the fire. 1
Seforno: The fact that, at Har Sinai, Hashem raised all the people to the level of Nevi'im by letting them hear the Voice of Hashem directly 2 is proof that He will never forget the covenant that He swore to the Avos.
Ramban: Precisely in order that His fear should be forever 'on their faces' to prevent them from sinning - and the other unique favors that He performed on their behalf, as the Pesukim go on to describe.
Seforno: Thereby taking them to Him as His chosen nation. By the same token, the Seforno adds (in Pasuk 34), one or a number of individuals may have been rescued from Resha'im (in a miraculous manner), but never an entire nation, as Yisrael was rescued from Egypt. See also Ba'al ha'Turim there.
Rashi writes that from the earth to the Heaven equals from one end of the Heaven to the other. According to the Kabalah that the earth and Shamayim are spheres, the latter distance is twice the former!
Mizrachi: 'From one end of the Heaven to the other' means from the earth to one end of Heaven (e.g. east or west). Maharsha (Tamid 32a)