By pneumatic counterbalance I assume you mean an air cylinder.
load to be countered= L 100 lb. ex.
Area of piston in cyl.= A 2" dia. piston (radius squared times pi) = 3.14 sq.in. ex.
Pressure of air in cyl.= P L (100 lb.) divided by A (3.14 sq.in.) = 31.8 psi air pressure ex.