You should buy a variety and try them out to see which fits best. Some of the size is more comfort than effectiveness. So this would depend completely on your own personal preferences.
There is no size chart that you can consult. The average ones fit almost every man; if you find these too loose or too tight, then you can try another size. They do fit almost everybody, however.
Although this seems like a funny topic, it really is important. Most condoms will fit just about any man. If you find yourself in a situation where your partner is much larger than average, then you need to purchase the ones designed for very large men. Otherwise, feel free to choose any style, flavor, color, or brand you like.