In search for a tiny Rf module for "spy pen" project.

I m searching for the tiniest rf module that i can find. The module should fit inside a fountain pen, 2.4 or 434mhz ? Should i consider custom pcb ? target size of the project 5mmx1cm ( including a tilt sensor). Any ideas ? thx

I'm looking for the smallest possible COTS RF transmitter for use with Arduino.

This is pretty small: https://www.sparkfun.com/products/10534

These aren't very big either: https://www.sparkfun.com/products/152