Preventing accidents, while still maintaining comfort, is a concern of any patient wearing an external catheter. Condom catheters come in a wide range of styles and sizes and there does need to be some planning to make sure the sizing is right so the catheter will hold and not leak.
Left to their own devices, many men instinctively want to choose the largest size. This is no time for ego! Remember the purpose is to ensure a snug fit without being too tight or too loose. One is painful for all day use and the other may cause leakage.
Most companies measure condom catheters by diameter and they may be called Small, Medium or Large, but normally they will be referenced by the diameter in millimeters. Common sizes are 29mm, 32mm, 36mm and 41mm. Many sites for ordering condom catheters have downloadable files where you can cut out a calibrated strip and test wrap it around your penis for a proper fit.
It is a good idea to buy a couple of different sizes if the you feel like you are between sizes. They are affordable, so it is worth testing a box of 10 in two or three sizes before committing to a bulk purchase of a 100 ct box. There is no point in wasting money on an ill fitting male catheter that will leak or be too confining.
If choosing a proper fit seems difficult, it may be wisest to consult your urologist or doctor. It is very important that the flow of urine as well as normal circulation not be restricted in any way. Good luck in your fitting and choosing a proper condom catheter.