How do accumulators work and how do you size an accumulator for application?

Take a look at the HS study manual Outcome 3.6.1: Understand the difference between isothermal and adiabatic conditions (pages 3-45 through 3-49).  The outcome focuses on understanding the process of how to size and calculate the usable hydraulic volume from an accumulator.  In addition, the MHM study manual has several outcomes dedicated to hydraulic accumulators: Outcome 1.2.6: Understand the function of accumulators (pages 1-58 through 1-61); Outcome 3.8.1: Check and adjust the charge pressure in an accumulator (pages 3-48  through 3-50); and Outcome 3.8.3: Replace a gas-charged accumulator (Page 3-52); as well as some additional outcomes on troubleshooting and testing a hydraulic accumulator.  These two resources would be a great starting point.