Sample size used for the audit of a plan

[Guest ayler11]

I am an auditor of Employee Benefit plans within an accounting firm. We historically have audited plans up to about 400 participants. Recently, we were asked to propose on a plan with 2100 participants. What I would like to know: How many participants should I use as my sample size in order to get comfortable with the whole plan? Our reference material, PPC, discusses 25 and 60 as a sample size without reference to how many participants it is talking about. Can anyone share their experiences either from the audit or the audited standpoint? If you have any reference material I should refer to that would be appreciated. Thanks for your help.