MiaB creates 2048 bit dkim keys … I do not know if there is a way around this. There has been other discussion of this in the past … /me searches … and is successful. However, the solution is an unsupported modification so I would not propose it to the OP of this thread, but here it is anyways:
The instructions in the linked thread make no sense though. What is the point of running a script with the 1024 commented out? I think that should be just a bit differently worded, no? Another old thread offers this: