Agree with using seatguru, it gives you seat dimensions specific to the flight and seat you are considering.
As for what could go wrong, plenty. We went from Chicago to New Jersey for a wedding and booked AA “first class”. On the outbound they cancelled the flight an hour before departure due to weather, even though no other carriers cancelled. It was too late to cancel the hotel without penalty,so we were stuck. They then flew us into La Guardia instead, telling us the contract allowed them to impose the alternate route. Once in NY I couldn’t get a rental, so we had to take an Uber to Newark to pick up our reserved rental.
They cancelled the return for the same reasons, weather, even though, once again, other airlines were flying the same route with no problems. It took us 3 days to get a return flight home due to continued rescheduling, all out of pocket to us, and the flight back was back of the plane. They eventually refunded about 1/2 of the difference between coach and 1st.
Dealing with the AA terminal crew was a memorable experience. They clearly do not like their jobs and have no motivation to be helpful or friendly.
Domestic first class with AA doesn’t get you much. A less uncomfortable seat and faster check-in. Certainly no better service in the terminal and, when things mess up, you’re pretty much on your own. All their attention is rescued for high flying status.