I set up an appointment with my vascular surgeon. I met him and told him what the Fresenius people had said. He examined the fistula. He also used a device, similar to the one that is used for an echo cardiogram and checked the fistula with it. He said that the thickness of the wall of the fistula and the skin over it was fine and there was no risk of any rupturing.
He also added that yes, the size was big but given that there was no risk of rupturing, he would not like to do anything to it right now. He could reduce the size but then the flows may not be enough for good dialysis. He also said we had limited options for another fistula because my arms had been used up for previous shunts and fistulae.
He concluded that we should let the fistula be and assured me that there was no risk of an emergency situation.