Interestingly, Google Cloud DNS has a fixed limit of 1012 characters per resource record (counting the quote marks that separate fixed length 255 char TXT strings). They claim you can have 10,000 resource records in a resource record set, so over 10mb of data.
Golly! Well, the minimum practical RR size is 2 bytes for the name (a compression pointer), 2x2 bytes for class and type, 4 bytes for TTL, 2 bytes for length, and some data, e.g. 4 bytes for an A record: total, 16 bytes. So you can't fit more than about 4k records in a DNS response.
It is actually possible to have a zone with an RRset much bigger than that, and it will transfer successfully with AXFR or IXFR, and you can make the RRset bigger or smaller with UPDATE - but you won't be able to query for it... unless the server has some nonstandard shenanigans for returning partial data, as many of them do.
