I’m 5’11” (180cm) tall, and here’s what the legroom in United Airlines’ economy class is like once the guy in front of you has reclined his seat. He’d originally reclined his seat all the way, but I was able to talk him into doing it only halfway:
As you can see, my knees are touching the seat in front of me. If you’re 6 feet or taller, you’ll probably want to get upgraded to what United calls “Economy Plus”, where you get a little more legroom – about as much as other airlines’ economy class seats.