Excluding structural mechanical strength considerations, the determining variable is backpressure on the PSV. If the resulting backpressure is within the allowable limit for the particular PSV type, then the tailpipe size is OK. This minimizes the costs, and for vapor streams released to the atm, this maximizes the dispersion rate. Some engineers also consider other factors such as Mach No.,RhoV2, noise, etc. Most PSVs will never lift, so I don't often get concerned about the long term wear on the pipe. If there's a choke in the tailpipe, that's OK too as long as the PSV can tolerate the resulting backpressure.
From a structural perspective, don't overlook acoustic induced vibration (AIV) risks which are associated with tailpipes connected to a collection header. Use the simple equation in API 521 to calc the sound power level. This sound power level is a screening criterion for assessing whether there's a risk of AIV failure at the point where the tailpipe connects to the header. Tailpipes < 10" diameter are potentially exposed to this failure mechanism.
