LdPmtc,
If you are in the U.S., you should probably have a copy of Ten States Standards for Wastewater Design. This is referenced in many state codes. Another free download is the Unified Facilities Criteria (UFC, formerly TM-Technical Manuals) from the U.S. military. I can't recall the UFC number, but if you google UFC wastewater it should be easy to find.
In general, the pumps for a typical lift station (serving a population between 100 and 10,000) are sized for the peak flow. It is required to have one additional pump not used in the design, i.e. a pump in a duplex pump station is sized for the peak flow using only one pump.
There is typically no storage volume. Pumps are required to handle the peak flow without storage. The sump volume between the pump on and pump off level is sized using the cycle time of the pumps, usually 12 to 15 minutes.