I came across this today (looking for something else ::) ),
(http://www.jaycar.com.au/products_uploaded/productLarge_14674.jpg) (http://www.jaycar.com.au/productView.asp?ID=QM3781&CATID=24&form=CAT&SUBCATID=909)
MP3 Player/Radio, Jaycar don't advertise the exact size but with it not having a CD player they claim it is not as deep as most head units. Should be easier to hide than a normal CD/Radio head unit, just rip your CDs to MP3s and download to an SD or USB stick.
That's not an FE or an FC! It's an FM!! ;)
I've seen a similar unit that Eclipse did a few years back and they'd be lucky to be 1" shorter than a standard head unit.
As for hiding them, you just have to be creative, when I was looking at options for mine I came up with half a dozen places to hide the unit/speakers in the car to still remain stock looking.