It is measuring at 4.5cm in diameter, this football inspired Shiro football MP3 Player has been designed to be hung from your keychain and comes with inbuilt 80×48 resolution OLED screen and with either 256MB, 512MB or 1GB of internal memory as well as a built in 30 preset FM radio receiver and even a stopwatch function. It costs about $50 dollars.

MP3, WMA, WAV Playback
256/512MB/1GB/2GB Memory
Digital FM Tuner & Recording
Digital Voice Recording
Blue OLED Display
ID3 Tag Support
USB Flash Drive
USB 2.0 Full Speed
Stop Watch and Timer Function
Li-polymer Battery with charging via USB
Shirocorp

http://www.shirocorp.com/products/MP3/mp3_AS.htm