Consider duration of the flight and your body size too as they will determine how much space you need; passengers seating next to you may play big role as well. Personally, I like sort of 'bracing' myself where I can rest my legs/knees against seat in front of me (without causing any inconvenience to the person in it). I consider flights within 3-4 hours airtime be 'easy', they get 'hard' when lasting over 7-8 hrs. Also, don't drink coffee/tea on the plane, only bottled beverages unless you wanna play lottery with your health.
If you collect miles with the airline and esp if you have status due to SQM/SQS and over 60 y.o. then you may ask for a curtesy upgrade to Business class. I was bumped to Buss once for free when flying to HI on B747, due to status with the airline, it was 1K with United.