The Building Regulations (B1 – 2.8/A ) states that any window which is provided for emergency egress should have an unobstructed clear opening area that is at least 0.33m2 and must have a minimum dimension of 450mm high and 450mm wide. The same size of an average cushion.
Is it small windows? Yes it is. At least the London Housing Design Guide requires all habitable rooms to have a window area equivalent to at least 20% of the floor area, which is a sensible approach to the design. This should be a national rule as there are many individuals happy to speculate on it.