If anyone does build a airline shopping site that does offer the ability to compare flights based on comfort I will gladly start using it exclusively.
The only tricky bit to this is that thanks to mergers from airlines which didn't have this, and the time needed to roll out fleet-wide cabin refurbs, there are still planes flying for those airlines which don't have the dedicated extra-legroom sections (and the tiny regional jets simply don't get those most of the time).
When in doubt, swing over to the website of the actual airline, enter the date/destination, and in the list of flights you can click to pull up a seat map which will show you whether that section exists and if so which seats are in it (American calls it "Main Cabin Extra", Delta calls it "Comfort+" and United calls it "Economy Plus").
Seat width is not really something US airlines differentiate on very much. Some have wider seats in the international version of their extra-legroom economy (typically only one inch wider), but for the most part if you want a wider seat you need to buy a first-class or business-class ticket.
(I actually really really hate air canada for this. I keep forgetting that I need to go to the united checkin at SFO, not the Air Canada desk which is 11 billion footsteps away at the other end of everything.)
In case it helps, the United code-share flights are numbered AC[345]\d\d\d.
I've also had this happen with Alaska and United.
http://www.wsj.com/articles/jetblue-to-add-bag-fees-reduce-l...
Seatguru and most flight searches will show you the info, but you do need to pay for more comfortable seats generally. Even on JetBlue, I buy the "Even More Space" seats when available.
Both British Airways and Emirates A380s are ten-abreast in economy. But BA's seats are narrower, being the same physical seat as they use in their 787s, resulting in uselessly wide aisles in the A380.
It's a mess.
This would probably change if I flew with any frequency though.
Because that's what the rest of us would be paying for coach if all the seats were big enough to comfortably fit you.
EasyJet and other LCCs actually have a better proposition for leg room now, as you can pay for and select an extra leg-room seat at time of purchase.
(EDIT: pre-empting "there's no need for leg room on short flights", I'm talking about flights of up to ~5 hours)
[1] http://www.seatguru.com/airlines/Lufthansa/Lufthansa_Airbus_...
[2] http://www.seatguru.com/airlines/Lufthansa/Lufthansa_Airbus_...
[3] http://www.seatguru.com/airlines/Lufthansa/Lufthansa_Airbus_...