A lot of people think they are, and this causes problems. Too small, and condoms break. Too large, and they slip off. Either way feels far less comfortable than with a properly fitting condom. This ultimately leads to less usage.
Condom boxes have something called "nominal width" on the side. To fit properly, you or your partner need to take a precise measure of the penis circumference, then match that to the appropriate width:
http://www.condom-sizes.org/condom-sizes/suggested-condom-wi...
Note that you have to double check the comdoms. For instance, magnums list 58mm, but actually are 54mm at the base, which is the most important place.
Fitted properly, latex condoms will feel significantly better and work better.