Using a condom that is not suited for your penis increases the possibilities of breakage and slippage, which in turn increases the possibility of sexually transmitted infections, or unwanted pregnancies. Thankfully, someone invented custom size condoms. Now they are available... in Europe.
According to Joe Nelson, CEO of TheyFit, they are now legally available in Europe, approved by the European Union. Sadly, the FDA hasn't approved it yet. The FDA sucks. But if you know anyone in Europe, measure your penis using this TheyFit kit and get a lot. They do make a big difference. Or a small one.