When attempting to add a long(ger than 255 chars) TXT record into DNS/Bind for DKIM I found that samba-tool would throw a python error about an overflow. To get around this limitation split the data portion into (no more than) 255 chars parts enclosed in double-quotes (") with a space between them, and then enclose the whole data text in a single quote (').

eg:

samba-tool dns add ad_server example.nz selector._domainkey.example.nz TXT '"v=DKIM1; etc to 255 chars" "more chars;"'

I also found using the Windows RSAT DNS tool helpful.