As suggested above, check seatguru for the specific aircraft type, or search the respective airline program forums for advice on the best seats for your aircraft. Many aircraft (some 737s on CO or Airbus A319/A320 on UA, for example, and many regional aircraft with first class) have cutouts at the bottom of the bulkheads or even just a curtain separating the classes, which allow you to stow your bags and stretch out. Some bulkheads do not have a cutout and actually might restrict legroom to stretch out.

If nothing else, you can at least purchase Stretch (extra legroom) seating on Frontier.

Additional information from the airline websites:
http://www.frontierairlines.com/fron...ecial-needs.do
http://www.aa.com/i18n/travelInforma...nningAhead.jsp