Can I have a TXT or AVC record longer than 255 characters?
You may have more than 255 characters of data in a TXT or AVC record, but not more than 255 characters in a single string. This is inline with limitations you have for TXT, AVC and SPF.
If you attempt to create an TXT or AVC record with a long string (>255 characters) in it, BIND will give an error (e.g. “invalid rdata format: ran out of space”.) Strings in TXT or AVC records should be no longer than 255 characters. However to get around this limitation, per RFC 4408 a TXT or AVC record is allowed to contain multiple strings, which should be concatenated together by the reading application. In the case of use for SPF (using either TXT or SPF RRs) the strings are concatenated together without spaces as described below. Reassembly by other applications of multiple strings stored in TXT records might work differently.
ISC Knowledge base: AA-00356