Although airlines aren't required to adjust their seating arrangements for long-haul and international flights, several elements go into providing more room if you're traveling a long distance in the economy section.
Most international flights use larger aircraft than domestic flights because it's more cost-effective to take one large group of passengers a long distance than taking several small flights the same distance. The larger planes used on these long-haul flights might have an inch or two more leg room, but the seat width usually remains the standard 17 or 18 inches. SeatGuru.com provides information on whether your airline uses planes with more legroom for international flights.
Many airlines offer extended leg room in the first cabin of economy. While it is becoming quite common on international flights, even domestic flights sometimes offer the first few rows of economy as "economy plus."
International flights that use two-aisle planes tend to be roomier. They offer more head room and, because of the aisle structure, at total of four aisle seats. If you sit in the aisle of the middle section, you might luck out by not having a seatmate right next to you.