when i was looking at doing the same thing in my car, i measured that i needed 13"-14", and i barely had 13"..
the other issue is space--it gets very cramped in there very fast-- esp with the heatsync space requirements a i3 will require.
i would recommend against mounting your setup in-dash-- look for better, more open locations, like under the seats, or in the trunk-- this way, you should have less problems with heat build-up as well..
i have one in my install, but rarely use it-- if you use flash drives, and external hdd's as much as me(my personal car keys have 2 flash drives on them, and my work truck has 1 flash drive, and i have at least 3 externals i can think of..), then it is probably not needed.. though it does come in handy when i buy a new cd that i want to listen too right away (i buy cd's because i can't stand the horrible bitrate of downloaded music anymore..)..
though there is the 41hz.com amp9 kit(also offered in a prebuilt version) i have heard very good things of 41hz's amps, but haven't had a chance to listen to them..
there is also the many cheap 'motorcycle amps' on ebay that could work as well--once you take them out of the case, they might fit(though as i said above, i think it is a horrible idea to put a i3 in-dash, mostly due to heat dissipation)
otherwise, have a look around, and make sure to browse the FAQ's(if you look enough in the newb sections, there is a link )