With LCD display could be small.....I recollect the LCD clock pens from years ago that had the clock display on the side of a normal sized writing instrument.

But I agree with Jez....The package of the CC400 is nominally 8 mm from lead tips to lead tips. To put this on a circuit board 10 mm (1 cm) wide is not likely to work very well from an RF circuit design standpoint. Not saying it cant be done but you would definately be looking at a multi-layer board with components on both sides of the board. The data sheet "typical application" circuit diagram shows the attachment of some 15 capacitors, 3 resistors, 3 inductors, a crystal, a varactor diode, antenna filter plus the antenna connection.

My gut feel is that the circuit board for this design will want to be at least 20 mm wide and use multi layer for a successful outcome.

With regard to which processor to use...the biggest issue if to pick one that can properly support a 3 volt design. The cc400 is a 3 volt type part. The inteface to the RF chip has to be manchester encoded but this can be done in software easily becasue the bit data bandwidth of the RF chip is 9.6KHz or less.

Michael Karas