In the US, all three of the major carriers have some type of extra-legroom economy-class seat, and all three charge extra to book into it (though if you have frequent-flyer status with the airline you can often get it at reduced price or free).

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.