I know it doesn't sound too exciting from the headline, but apparently this is a "world's first" so I'm going to post it here.
The actual part that you can change the size of is the headphone chip itself. I guess if you've got a small/large ear canal, this would be the product for you.

Tohoku Pioneer Corporation will release the "SE-CS25," inner-earphones whose earphone chip size can be changed, o­n May 10th. The price is 2,300 yen. Three colors are available: Silver (-J1), Black (-J2), and Gold (-J3), where the Gold model is a limited edition.

Inside the container, o­ne will find "small" and "regular" sized earphone chips. It's made that so o­ne will choose the size that best fits their ear, and "gain a comfortable installation experience." While there are earphones available that have changeable earplugs (canal type), Pioneer has said that "this is the world's first for inner-ear type."

With a target market of males and females in the 25s-30s range, the earphones have a "simple and stylish" design. The weight without the cord is 6g. They use a 13.5mm diameter unit. The driver is a rare-earth magnet. Playback frequencies run from 18Hz ~ 22kHz. The impedence is 16 Ohms.

The connector is a gold-plated stereo miniplug. Although the standard cord is 50cm long, a 1m extension and spare earpads are included.



Inspired by:
http://www.watch.impress.co.jp/av/docs/20040412/pio2.htm

Tohoku Pioneer Homepage:
http://www.pioneer.co.jp/topec/