prepareToPlay is a method of AudioProcessor, not AudioDeviceIOCallback, so what mechanism are you using to connect the device manager to your plugin class?
This is a strange one. It seems that the iOS audio classes just always make the callback using a 512 byte buffer, regardless of the buffer size that the device is using.. Really not sure what can be done about that, the buffer size is definitely set correctly.
(BTW in looking into it just now I spotted and fixed another bug where it seemed to fail to set the correct buffer size)