Since I'm probably going to book one of the big Chinese airlines for a long haul flight, here are their longhaul seat pitches in Economy
I'm using AeroLopa - which usually has the most up to date information
First off there's China Eastern
Their twenty A350s have a 3-3-3 layout and 31" pitch and 18" width
Their 20 B777s have a tighter 17" seat width in a 3-4-3 layout but with 32" pitch
They have two different A330-300s configurations - with either
a 17.3" seat width in a 2-4-2 layout and a 31" pitch or
a 17.8" seat width in the same 2-4-2 layout but with a 32" pitch
They have three different A330-200 configurations - with either
a 18" seat width in a 2-4-2 layout and a 32" pitch or
a 17.8" seat width in a 2-4-2 layout and a 32" pitch or
a 17.2" seat width in a 2-4-2 layout and a 32" pitch or
Next up is Air China
Their fourteen B787s have 17.1" width in a 3-3-3 layout and 31" pitch
Air China's six B747-8s have a generous 18" width in a 3-4-3 layout and 31" pitch
Their 28 B777s have 18" width in a 3-3-3 or 3-4-3 layout and 31" pitch
Their 29 A350s have 18" width in a 3-3-3 layout and 31" pitch
Their 28 A330-300s have 18" width in a 2-4-2 layout and 31" pitch
Their 21 A330-200s have 18" width in a 2-4-2 layout and 31" pitch
And finally China Southern
Their twenty A350s have 18" width in a 3-3-3 layout and 31" or 32" pitch
Their ten B787-8s have a 3-3-3 layout and 31" pitch and 17.3" width
Their twenty B787-9s have a 3-3-3 layout and 32" pitch and 17.3" width
Their fifteen B777s have a 3-3-3 layout and 32" pitch and 18.5" width
Their 22 A330s have 17.4" or 18" width in a 2-4-2 layout and at least 32" pitch
Conclusion
Avoid China Eastern's twenty 10 abreast B777s
Avoid Air China's twenty eight 10 abreast B777s
China Southern don't have any 10 abreast planes
Online Checkin
China Eastern opens online checkin 48 hours before departure
China Southern does not allow checkin from most foreign airports
Air China opens online checkin 36 hours before departure for direct bookings only