You'd be better off in the Safety Relief Valve Engineering (PSV) forum.
Typically the inlet is smaller while the outlet is bigger (3/4x1" valve, 2x3" valve, etc). If you're asking for pressure drop criteria that may be found in ASME BPV Section VIII and API 520. Not sure what you're asking about with regards to the "philosophy" of the sizes.
IMO, the sizes of the PSV inlet and outlet are calculated and optimized by the actual flow rate through the valve effective orifice as listed on API-526.