Hello,
I assume you are trying to implement DKIM in Google Cloud DNS as you have created this issue in cloud-dns-discuss Group.
As per our public documentation[1], there are three kinds of TXT records including DKIM that could be used to secure your e-mail services and prevent spammers and scammers from sending an e-mail that appears to come from your domain.
Regarding your issue on character limit on TXT records and requirement to select the smaller 1024 key, as per our 'Update DNS records for a subdomain’ documentation link[2]:
If your domain provider limits the size of the TXT record value to 255 characters, you can't enter the DKIM key as a single entry in the DNS records. In this case, split the key into multiple quoted text strings and enter them together in the TXT record value field. For example, split the DKIM key into two parts as follows:
"k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAraC3pqvqTkAfXhUn7Kn3JUNMwDkZ65ftwXH58anno/bElnTDAd/idk8kWpslrQIMsvVKAe+mvmBEnpXzJL+0LgTNVTQctUujyilWvcONRd/z37I34y6WUIbFn4ytkzkdoVmeTt32f5LxegfYP4P/"
"w7QGN1mOcnE2Qd5SKIZv3Ia1p9d6uCaVGI8brE/7zM5c/zMthVPE2WZKA28+QomQDH7ludLGhXGxpc7kZZCoB5lQiP0o07Ful33fcED73BS9Bt1SNhnrs5v7oq1pIab0LEtHsFHAZmGJDjybPA7OWWaV3L814r/JfU2NK1eNu9xYJwA8YW7WosL45CSkyp4QeQIDAQAB"
[1] https://cloud.google.com/dns/docs/dnssec-advanced#spf-dkim-dmarc
[2] https://support.google.com/a/answer/177063?hl=en&ref_topic=2752442