MCU selection advice

I need to find a MCU for a small project which need only one digital input and one digital output.
The digital input need to have external interrupt capability. The MCU must have internal timer for controlling up to 30 mins and interrupt capability.

Project will use battery and lighting 4 LED only.

Can advice what are smallest Atmel MCU shall i choose to work on ?

Sounds like a job for an ATTiny85 (or one of the smaller variants thereof, ie, tiny25/45, depending on how small you can make your code)

Even smaller is the ATtiny13 - in capacity that is.
Don't know of those older models are available in QFN package, which can bring the physical size down to 3x3 mm.
Try this selection guide: http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=1005