I have More’s not Reals, but I think most of this will be the same.
I think to use the iPhone’s mic you open the control center. When you click on the ear icon to get to the hearing aid controls, do you have an option for Mic Input? If you switch Mic input to off does it use your iPhone mic? I think this should work but I am not certain (I can’t verify right now)
Sorry, I have not had the volume issue, mine is loud enough (and does change with the phone volume controls)
Wire size - I’m not sure if there is a way to measure, but for me it was trial and error. She used size 1 and when I tried them on she told me they were too long and changed to smaller ones. So I am no help on that one either.
Sorry