Here’s an application note from Atmel on microstrip layout techniques to get optimal RF performance from wireless ICs (PDF!) :
Poor layout can seriously degrade wireless performance. This application note outlines practical microstrip layout techniques to connect wireless microcontrollers with baluns and antennas. Unbalanced and balanced (differential) microstrip techniques are discussed for 4-layer PCBs using FR-4. Other design topics include PCB stack-up and ground layer practice, QA and PCB production strategies.
The width of the tracks is based on the dielectric constant of the PCB material.
Closely followed by the thickness of the PCB.
Get both of these from the manufacturer, before you start!
[double check copper thickness while you’re at it…]