The value field of a DNS record can not exceed 255 characters. As a result, it is not possible to create a DKIM record with a 2048 bits key cause it requires 411 characters.
As stated if RFC 1035, maximal length of a TXT DNS records is 255 bytes. OBAS handles such kind of records with length not greater than 255 bytes without errors.
However to get around this limitation, per RFC 4408 a TXT or SPF record is allowed to contain multiple strings, which should be concatenated together by the reading application.
3.1.3. Multiple Strings in a Single DNS record As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS record (either TXT or SPF RR types) can be composed of more than one string. If a published record contains multiple strings, then the record MUST be treated as if those strings are concatenated together without adding spaces. For example: IN TXT "v=spf1 .... first" "second string..." MUST be treated as equivalent to IN TXT "v=spf1 .... firstsecond string..." SPF or TXT records containing multiple strings are useful in constructing records that would exceed the 255-byte maximum length of a string within a single TXT or SPF RR record.
The feature request PBAS-29730 has been submitted to implement RFC 4408 specifications in future OBAS versions.