Above, I've amended the casualty figures for the far right on 6 February 1934 ("14 or 15 killed, 1500 injured") in the printed version, got from checking online sources, to "16 killed, 600-odd injured", now that I've been able to get copies of Robert Soucy's French Fascism: The Second Wave and Serge Berstein's Le 6 fevrier 1934, which give that information.
The police shot and killed six on the Communist Party and Socialist Party counter-demonstration which followed on 9 February.
Soucy estimated 40,000 for the 6 February 1934 crowd, though Berstein and most other sources say 30,000.
Barrie Hardy in Solidarity gives 40,000 for 6 January 2021. Scanning the serious bourgeois media in the USA, I've been unable to get any estimate beyond "thousands". Photos look to me like more than 40,000 (and much more than the 15,000 I've seen quoted in some places).
Of course the USA has a bigger population than France in 1934, but in comparing turnouts we must also consider that 6 January 2021 demonstrators mostly had to travel long distances in the middle of a pandemic (Trump has only marginal support in Washington DC, and not very much in the whole eastern seaboard), whereas the far right in France in February 1934 was strong in Paris (Soucy gives figures).
One thing omitted from the article for lack of space is that the Communist Party's army-veteran front, ARAC, supported the 6 February demonstration. Improved war pensions were a major motive for many demonstrators.
Soucy reckons that Coty's Solidarité Française was bigger in February 1934 than the Croix du Feu, but since SF collapsed within weeks of February 1934 (Coty's money dried up) and many of its members (so Soucy says) joined the Croix du Feu, the reference to the Croix du Feu as the biggest far-right group can stand.