What is the pressure in a civil aircraft fuselage at flight ceiling?
The pressure inside the cabin is usually more than the outside pressure at the cruise altitude. The pressure altitude (ISA altitude with the same pressure) inside the cabin of civil aircraft is called the equivalent effective cabin altitude or normally cabin altitude. 

As the aircraft climbs, the cabin altitude is gradually raised so that it reaches around 8000 ft for the aircraft cruising altitude of ~40000 ft. This level is maintained at flight ceiling too.



Source: www.flyertalk.com

The pressure is around 0.75 bar inside the cabin. The outside pressure at this point is around 0.2 bar, resulting in a pressure differential of 0.56 bar. This pressure differential causes stresses on the aircraft body.



Source:ecnmag.com

In addition to the stresses, pressurization - de-pressurizatoin cycle can cause fatigue in the windows.

As reducing the cabin altitude will improve the passenger comfort, newer aircraft tend to have lower cabin altitudes. For example, the Boeing 787 has a pressure altitude of around 6000 ft and the Bombardier Global Express jet has a cabin altitude of around 4500 ft.

This is achieved by the use of advanced composite materials, which allow a higher pressure differential have excellent fatigue properties compared to the metals. As a result, the windows of newer aircrafts are considerably larger than their predecessors.


Image: Boeing