This may be not exactly an answer, but it may be useful. I have come upon a case of a leader's use of “Dictator” in 1898, later than Garibaldi, and as an official title, rather than a more informal self-description as has been shown of Presidents Lukashenko and Bukele of modern Belarus and El Salvador: Dictator Emilio Aguinaldo of the Philippines (Wikipedia). Wikipedia cites the National Historical Commission of the Philippines, which says that:
- Aguinaldo led a “dictatorial government” from 24 May 1898, using the title “Dictator” until he changed it to “President” on 23 June 1898;
- “By July 1898, the Filipinos liberated the provinces of Cavite, Laguna, Batangas, Pampanga, most of Bulacan and the suburbs surrounding Manila”.
The first four provinces — Cavite, Laguna, Batangas, and Pampanga — have, within their modern borders (as of 2007), a total of 867 424 hectares, or 8674.24 km², more than 3.35 times the 2586 km² of Luxembourg.
However, with Aguinaldo's time as "Dictator" lasting from 24 May to 23 June, he fell
one day short just over 50% short of the two-month minimum, and he didn't spend all of that time in control of all four of those provinces. On the other hand, if it is acceptable that the leader call himself a dictator at one time and control a Luxembourg-sized area for two months at a different time, then Aguinaldo may qualify still.
Someone familiar with Philippine history (which I am not) may be able to provide more detail (and maybe better sources than an understandably patriotic Web page of the Philippine government).
-  "The First Philippine Republic", 7 September 2012, National Historical Commission of the Philippines, https://web.archive.org/web/20211007035227/https://nhcp.gov.ph/the-first-philippine-republic/
-  "List of Provinces", 31 March 2016, Philippine Standard Geographic Code Interactive, Philippine Statistics Authority, https://web.archive.org/web/20160419000512/http://www.nscb.gov.ph/activestats/psgc/listprov.asp
-  "Luxembourg", 28 December 2021, The World Factbook, U.S. Central Intelligence Agency, https://web.archive.org/web/20220112011745/https://www.cia.gov/the-world-factbook/countries/luxembourg/