That's the standard size for an aux input.
I got a new Honda Fit Sport (LOVE IT!!!), but have no idea what kind of cord I need to connect my iPod into the car. The car salesman told me a 3.5mm adapter, but do I need a 3.5mm to a 2.5mm or a 3.5mm to a 3.5mm? If anyone knows the answer, I would greatly appreciate any info! Thanks!!