Ethics mean we don't know what the average human male erect penis size is.
No, really. The ethics of the studies say that a researcher can't be in the presence of a sexually aroused erect penis. Having the testee measure their own penis is prone to error. There are ways to induce an erection with an injection, so they use that.
Is the size of an induced erection the same as a sexually aroused erection? Probably in the same ballpark, but we don't really know.
Source: Dr Nicole Prause, neurologist specializing in sexuality, on Holly Randall's podcast.