Spending four months in Chile is something I would have never been able to do without fly fishing. I have some very seriously cherished memories from my time spent there. That said, I think expectations need to be managed for trout sizes in that region. The lodge I worked for was in the Aysen region which is about a two and half to three hour drive from the nearest "city" of Coyhaique. We primarily fished the Rio Cisnes which is yearly in Patagonia's best rivers to fish, and to be completely honest I really was not that impressed. In my opinion it fished about as well as some of the lower tier Montana rivers. There was plenty of fish, most days, but there was no real size to the trout. The average was probably 12-14 inches. The winds were 98% of the time up river blowing 30 mph and sometimes gusting up to 60 mph. So much so that on several occassions I could only have one client casting at a time to avoid tangling and hooking each other.. and me.
All that being said there are some big fish in Chile, you just need to find the right places, and be there at the right time. The big deep rivers where you are ripping streamers and sink tip all day. Or the lodges that have expansive spring creeks with tons of terrestrial insects (who also get a special diet of pellets) are your best bet. There is also a place on the Argentina side called Jurrassic Lake, if you get a chance to look that up you will be thoroughly impressed with the size of the trout over there.
If I were to ever go back to Patagonia I would make sure that I budgeted some time to see the sights. The country is absolutely breathtaking. I wouldn't trade my experience for anything and hopefully someday I will get to go back.. Crazier things have happened.