Biggest Condoms in Japan
Recently I was in a supermarket and found the condom section. In the past I had some issues with Japanese condoms since they were just too small for my western-sized pecker.
When I met my Japanese wife many years ago, one of the first parcels she received from well-meaning relatives was a packet of condoms. We found this strange since Japanese women are taught by blokes that love-juice helps to keep their skin young looking so they tend to milk you and impregnation is mostly for making babies. Anyway, we gave them a try but they were useless so I went back to my old favorite Mates brand.
Back on topic, I discovered that the condoms are now precisely defined on the packet by diameter in mm. So all you need to do is get a tape measure and measure yourself when hard. I suggest going to your local tailors and asking the female assistant for help with this since they are likely to have the old flexible tape measure like they use for inner thigh and waist measurements.
Armed with your measurement, you then need some math. The condoms are specified as large= 38mm and ultra large= 44mm diameter. So you need to work out how to convert the girth of your manhood to the diameter. This is not easy since it is not a simple circle to diameter calculation so you are best advised to ask the counter staff for help in determining the best fit for you.

