Hello @robertw7630 ,
At this time, 1024 is the only key available for self-authentication with us. I've marked your account as interested in having 2048 available, and submitted it to our deliverability devs. If and when there's an update to the existing feature request, they'll notify you directly. At this time there is no ETA on the addition of the more complex keys.
If you have additional questions, please reach out to our Deliverability team.