really small arduino power supply

The requirement was definetly a minimum, more the merrier, this is for a design for somethi that is going to be behind regular house outlets and switches(probably in the same box, so im trying to keep it cool and small as possible, basically going to be powering an attiny84/85, relays, leds, perhaps some other comm peripherals
so while quality isn't the most important, reliability definetly is, something that could be expected to last perhaps atleast 3 years by itself in a closed enviroment, without gtting hot or posing a fire hazard lol