Sizing of a pressure relief system depends on what the potential causes of overpressure in the system to be protected are. For example: thermal expansion, fire exposure, chemical reaction, pumping into a pipe or vessel with all outlets closed or blocked, etc. You can find a good introduction to sizing of relief devices, including simple equations for certain scenarios, in Crowl and Louvar, Chemical Process Safety Fundamentals with Applications, Prentice-Hall, Upper Saddle River, NJ, 2002, ISBN0-13-018176-5, www.phptr.com, Chapter 8.

Also, there is a much more extensive discussion in the 3rd Edition of Lees' Loss Prevention in the Process Industries, published by Elsevier, Chapter 12. There is also an extensive discussion in Perry's Chemical Engineers Handbook, starting on page 26, which covers the entire process from scenario selection through the design equations.