Worth noting that it's 4K after DNS compression, so you can fit a crapload of _acme-challenge records in there. Something like ~70.
To save someone trying, I setup two zones, one under 4K (under4k.foo.monkas.xyz), and one over (over4k.foo.monkas.xyz).
First gives:
Detail: Incorrect TXT record "5105jgUpOBEFaxu7MNo8ifu8zslVWRZJ9tuxH8kXA8k" (and 68 more) found
at _acme-challenge.under4k.foo.monkas.xyz
Second one:
Detail: No TXT record found at _acme-challenge.over4k.foo.monkas.xyz