It's a pretty simple bug, but it's related to Aric's mod not vanilla. Aric forgot to increase the clamped index range when adding his new sizes to sexdescription.gd. In line 1250 it says:
return getRandStr( descPenisSize[ clamp(limitsPenis[0],0,2) * 2 + int(areAllAttrib_E_val(group, C_AGE, C_CHILD)) ])
but it should be:
return getRandStr( descPenisSize[ clamp(limitsPenis[0],0,5) * 2 + int(areAllAttrib_E_val(group, C_AGE, C_CHILD)) ])
It just changes the max size from 2 to 5.