I would suggest the Zeppelin LZ 24 "L3" of the Imperial German Navy. It had a length of 158m and a volume of 22,470 m3.
Now admittedly it was not intentionally flown without a pilot on board. On 17 February 1915 L3 was escorting a German ship heading for Africa, but after an engine failure compounded by strong headwinds and lack of fuel the captain was forced to put it down on the island of Fanø, Denmark (just north of the then current border with Germany)
The wind was so strong it blew the airship, now
unmanned but with engines still running, out to sea.
Eventually it crashed (but so did the Boeing 720 mentioned elsewhere!). Kapitänleutnant Hans Fritz and his crew were detained for the rest of the war. This airship had the distinction of participating in the first air raid on England on 19 January 1915, killing two residents of Great Yarmouth.