It may only be the size of a thumb, I think that's a bit on the small side, but the food moves through rather quickly so you can eat more than that amount. Instead of thinking," How much can I eat?" I always tried to thi k " How little can I eat?"
In other words, from the very beginning I ate like I was in the elusive "green zone". I measured out my 3 or4 oz of protein and 1/2 cup veggie and when that was gone I walked away, that was it. And eventually, I was totally satisfied with that amount!