How small do computers get nowadays?

[Proginoskes]

Hmm, this is tricky. iDevices don't have any USB host capability at all, unless you jailbreak, purchase a separate dongle (the "Camera Connection Kit"), and buy a $1 Cydia app. Even then, there are compatibility issues: input devices like keyboards and mice work, as do storage devices like hard drives, but printers? Not so much, unless you're willing to do a lot of raw I/O grovelling and driver-writing. On top of all that, an iPhone 4 without the CCK dongle is over 115 mm long: way too big for your box. Arduino devices are essentially especially easy to work with microcontrollers, and both are cases of "probably more trouble and expense than it's worth", but might still be a better choice than an iPhone. Without doing any real research, I'm going to guess that using an Android device would be a similar hassle. BeagleBoards are complete computers (like a low-end desktop) just slightly too large for the box. You might be able to find a prebuilt print server that'll fit... but I think you're working with slightly too little space.