What makes it Arduino compatible is running it at 16 MHz so an Uno bootloader will work (or 8 MHz and a different board type, like 3.3V Promini).
There are several small versions of cards - the Promini, other surface mount variants (like mine, with offboard USB adapter needed and crystal and some Rs/Cs on the back side), and even Femtoduino, using the leadless Atmega chips and smaller pitch connector holes.
http://www.femtoduino.com/
I see Picoduino on google, using Attiny85 chip. IDE can be added to, to support the chip.