[EricH]

Just for reference, 2.5" > 2".

With that being pointed out, let's go through the math on exactly how to determine the bore size needed.

We are attempting to lift 150lb of robot. (Nice round number, little bit big but that's OK for now.) We can only use 60 psi air. So, we need a relation something like this:

150 lb = 60 psi *A in^2

Solving for A, we get the following relationship:
A = 2.5 in^2 = pi*r^2.

r^2 = something like 0.795 in^2, and 2*r = diameter, which is what pneumatic cylinders usually are sized by. Some quick math returns that a 1.75" cylinder will be too small, while a 2" is big enough--note that this is from a pure force standpoint, and neglecting the area occupied by the rod part of the cylinder.

The thing with pneumatics is that you want to use the smallest cylinder you can get away with, in general (at least in FRC).