At this time, it is believed that a 1024 bit secured DKIM record is insecure. See the following article from the National Institute of Standards and Technology:

https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar2.pdf#page=15

The current option of 1024 bit DKIM records is going against industry best practice. Here is your reference knowledge base article:

https://www.kb.blackbaud.com/articles/Article/103756

Please allow the organization to choose 2048 bit keys.