(As for cases when the evidence from race is not so easy to screen off... well, I've never heard anybody being accused of racism for pointing out that Africans have longer penises than Asians.)

Minor note, this appears to actually not be the case. Most studies have no correlation between race and penis size. See for example here. The only group that there may be some substantial difference is that Chinese babies may have smaller genitalia after birth but this doesn't appear to hold over to a significant difference by the time the children have reached puberty. Relevant study.