I think you're going to be surprised.

A Boeing 747F has a fuel capacity of c.200,000kg, 200 metric tons (http://www.airliners.net/forum/viewtopic.php?t=738927).

It's mostly heptane (C7H16) and octane (C8H18). Rough calculation, that's 15x12 : 34x1 ratio of C to H (180:34).

So let's say 86% by mass, of 90% is Carbon.

But CO2 is mostly Oxygen from the air, about 12:32 ratio. So the Carbon is a quarter of the total mass.

This gives us (0.86 x 0.90 x 200tons) x4 as a ball park figure.

That's 600,000kg of CO2 produced by 200,000kg of fuel.

With 400 passengers that's 1500kg of CO2 per passenger if they burnt the full fuel capacity; ie that's a top bound estimate.

So ...

Meanwhile you might be surprised to find trees are made of CO2 with the oxygen removed. A large oak tree weighs c.5000kg and half of that is Carbon.

So very, very roughly you'll want to plant and grow to full size a large tree for every couple of long haul flights (to account for fuel only).