Is Arduino board too big? Check out Femtoduino

The main issue about making a CanSat is the limited space you are given. Therefore, the main ability of the engineering team is optimization.

If you are having problem with space, there is something you should take a look at: Femtoduino. Femtoduino is an ultrasmall Arduino compatible board. Its incredible dimensions are just 20.7x15.2 mm. Its base is an ATMEGA 328p. Despite the fact that some components have been removed, this lightweight board has the same power of the Arduino Duemilanove or UNO.

As the CanSat  board is built onto an Arduino board, you might want to replace the original board with this one. Femtoduino is a home-made prototype, but if you are an advanced user, take a look at the official project page below. 

You also have a video showing the main points of the board here:

Via Femtoduino

This article in Spanish