Ok. The German school system is very different from some others so I will share the knowledge, bestowed upon me, with you.
1. Most men have a penis which size falls into a very narrow range and therefore standard condoms fit most pretty good. However the right size is very important, because too big condoms can slip or fall off and then they may get lost inside your partner. Yes, that happens and it's very awkward to explain to you gyno. https://static.lushstories.com/emoticons/lush/forum/eusa_whistle.gif Condoms that are too small are more likely to rip. Both scenarios render them useless.
2. Most quality brands offer sizing guides or online size calculators. So you just measure the erect(!) penis both length and girth and then check for the right size. Here is a overview over some brands: http://contraception.about.com/od/malecondom/a/CondomSizeChart.htm
3. You should invest in quality condoms because it's your body and your health. But more important is the right storage. They are very sensitive against temperature and sun. So store them at steady room temperature and keep them away from direct sun light. Btw always check the best before date!
4. They do come in different materials, which one you chose is a personal choice for most people. They also come latex-free so an allergy is no reason to not use a condom.
5. It's also important to handle them right. Open the package carefully and keep long fingernails away. The right way to take them off is not to roll them back up like many people do, because you can have spillage. Instead you have to slide them off and make a knot in the end immediately.
6. Trial and error might not be the best way to find out the right size. However you can use it to find out your favorite brand.
7. I always have an assortment of condoms at home, because I have to agree with Dani. You should be prepared.

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