What penis girth is too small for "normal size" condoms?

Simply put, if it fits snuggly when the penis is erect, it is a good fit. The majority of penis sizes will fit standard condoms. I am curious as to why you are asking this question and if you have concerns about an abnormality you should see your provider and have that discussion.
Condom size. Regular condoms usually work for a girth btwn 4.7 to 5.1 inches. If girth is less than 4.7 inches (119 mm) may need a smaller size. See this site for more detailed information: http://www.condom-sizes.org/condom-size-chart/condom-size-chart.