The adjective of size is another word of universal kindred, though not always with the same meaning.  The Sanscrit mahat, and Persian mi or meah, are close connections of the Gothic mikils (which survives in mickle and muckle, and has furnished our much), and of the Greek μεγαλος or μεγας, and Roman magnus and Slavonic magi.  All these possibly may be remotely connected with the verb magan (may), which is the source of macht (might) in all Teutonic tongues.
    Magnus was an agnomen added as a personal distinction, as in the case of Pompey.  It was never a name till long after the Roman empire was over, when Karl der Grösse, as his Franks called him, had been Latinized into Carolus Magnus, and honoured by the French as Charlemagne.  St. Olaf of Norway was known to be a great admirer of Charlemagne, whose example he would fain have imitated, and his followers, by way of a pleasant surprise and compliment to him, before they woke him to announce to him the birth of his first son, christened the child, as they thought, after the latter half of the great Emperor Carolus Magnus.  That child became a much-beloved monarch, under the denomination of King Magnus Barefoot, from his having established his identity on his return from Ireland, by the ordeal of walking unshod over red-hot ploughshares... The Scottish islands, where the population is Norse, likewise use Magnus as a baptismal name; and the Lapps have turned it into Manna, or Mannas. (History of Christian Names, Yonge, 1884)