Recently, US-CERT published a note pertaining to a weakness about DKIM keys when those keys have a length of less than 1024 bits (reference).
This weakness allows a low-cost attach to deduce a private key with very little effort: as an example, Google’s private DKIM key has been discovered in only 72 hours using Amazon’s Cloud Computing services for a total cost of $75!
In response, it is now recommended to only use keys that are 1024 bits or stronger, and most DKIM validation services are rolling out updates that will ignore any DKIM signature made with a 512 or 768 bit key.
INTELLIGENT EMAILS, REP Solution, along with most entities like Google that has to sign a very high volume of messages, originally generated 512 bit keys for its customers while setting up accounts. If you use DKIM signatures and your keys have an insufficient length, you will be contacted shortly by our service team. A new set of 2048 bit keys will be generated and the public part of those keys will be sent to you, to be added to your DNS servers.
Once these keys are published, INTELLIGENT EMAILS and REP Solution’s server will start signing messages using the new keys.
As a complement, DNS official specs says that one TXT record can be as long as : 65535 characters!!