Keep it Small!The DC Boardino and USB Boarduino are the most Uno Compatible options. They are about 1/3 the size of an Uno but use the same Atmega 328P processor. Your Uno sketches should run without change. The Boarduinos can be built without sockets or headers for a lower profile. The DC version can be made even more compact by eliminating the voltage regulator section of the board if you already have a regulated 5v supply.
The Flora is both small and thin! It uses the same processor as the Leonardo, but takes up less than half the area.
The smallest of all is the Micro. This is about 2/3 the size of the Boarduino and also uses the 32U4. Your Leonardo projects should easily port to the Flora or Micro.
Not Too Tall!A standard Arduino with headers is pretty chunky, and plugging things into the headers only makes it bulkier.
The Leonardo is available in a headerless version. and the Flora has no headers either.
Both the DC Boarduino and the USB Boarduino can be built without sockets or headers to make a thinner package.
Integrate!Sure you can switch to a tinier processor, but what do you do with the rest of the electronics? Short of a custom circuit board, there is the Adafruit Menta with its built-in prototyping area.
The Menta is sized to slip into a mint-tin for a simple and attractive package. But they are also great for any embedded project that needs some custom circuitry.
The Menta uses an Atmega 328, so it is compatible with the Uno, and it has standard Arduino headers for excellent shield compatibility!
Wear it with Pride!While many of these compact processors could be worked into a project you could wear. The Flora is specifically designed for wearable projects.
In addition to the thin profile and sewable connections, these processors are designed with rounded shapes that are fabric friendly with no protruding pins or sharp edges to snag on fabrics.
With their attractive appearance and selection of similarly styled Flora peripherals, these are the boards to go to when you want to wear your microprocessor on your sleeve.
The Flora is based on the Atmega 32U4 processor, so it is largely Leonardo Compatible.