A condom should be comfortable, covering the entire penis with comfort and leaving room for air in the tip. In addition to being extremely uncomfortable, a tight condom runs the risk of rupturing during sexual activity.
However, if a condom is too long, it will roll up the shaft during sex and can end up hanging off the penis. Not only does this render the condom useless, but it can also sour the situation when you have to pause and take it out.
The ring that surrounds the condom's base could break if it is too short. A condom that is too short risks having its base ring break during sexual activity, which could cause the condom to come off or rupture. This may lead to a reduction in the condom's effectiveness and make STI transmission possible.
According to a 2021 study, the average erect penis length is between 5.1 and 5.5 inches, while the average girth is between 4.7 and 4.8 inches. If you've heard that the average length is at least 6 inches, that isn't accurate anymore.
Here’s a table for those with a longer and wider penis. Courtesy: Greatist.
Durex Thin Feel Ultra Thin 7.08 2.04″
Trojan Ultra Thin 7.5 2.09″
Durex Pleasure Me
Ribbed & Dotted 7.68″ 2.2″
Trojan G-Spot 7.9″ 2.09″
Durex Comfort XL Large 8.66 2.24″
Trojan Magnum Ecstasy 8.3″ 2.09″
Below is a graphic for those with smaller penis.
You can measure your penis yourself with a tape rule when erect to determine the size or you can ask your partner to do it for you.