Why not?
Just use the RF to EF adapter instead. That`s what I'll do, provided the camera is small enough.
Because that would force the use of EF-glass on the RF camera and would mean that two adapters are needed.
I was referring to the case where the "main" camera used to be EF-mount camera, has been replaced by an RF-mount camera, and a M6 or M6II was already there as a back-up camera. The latter can no longer be used as a back-up camera for the RF-mount camera, except if the RF-mount camera is used with EF glass only, and with the RF-EF adapter.
This was a response to another post which stated that the M-camera can be used as long as it doesn't break.
Of course if the rumoured new RF-mount camera happens to be almost as small as the M6II, it could serve the same purpose. I don't see where the RF to EF adapter would be needed though, or were you referring to the case where the RF-mount camera would be used as a back-up for an EF-mount camera?
As for size expectations, technically the camera could be almost as small as an M6II, but that would compromise ergonomics and Canon is not known for doing that. At least the RF-S lenses are almost as small as the EF-M lenses, so the complete package might still be very small