There is no trunk in the Smart. You have to remember its 1/2 a car. ;-) It does however have a small area above the engine cover between sets and the tailgate. A box could be made for large speakers. I think a few have done so. There is some area behind the seats for amps or under the seats for small profile amps. The alternative is a sub box that fits under the dash on the passenger side.
Myself have 4 Polk 6.5" 2-ways and the JL 8" sub under dash. Powered by a Alpine PDX5 under the seat. No head unit. Full Carputer behind the seats. It is more then loud enough. I need to find my db meeter, I'm sure I can push more then 120db. At 40% your ears start to ring. Not as baseie as my truck with 4 5.25" 3-way cerwin vega's and a 10" Polk sub powered by 400watt Kenwood amp.
You could prolly fit 2 10" in the back in a box. But 12" you are libel to pop a window out