Designing a small PCB for the cc2540

I have made a pcb for the cc2540 that is 9mm wide and uses all of the I/O pins. So you should be able to fit it on a 20mm diameter PCB no problems but you will need 4 layers (at least). Keep in mind you will need some PCB space clear of the battery and other components for the antenna.

I'm NOT a hardware guy :)

But what if you simply bend the hardware keyfob fo put the circuitry below the CR2032?

The printed antenna is more or less the same size.

It will not be the most effective radio design but I guess this could be done.

Or what about the EMK modules? They are quite small on theirselves, I do not think it would take long to reduce them a bit.

Perhaps this could help you out.  http://blueradios.com/hardware_sensors.htm  BlueRadios has created a board that is slightly bigger then the CR2032 and has their CC254X module, light sensor, accelerometer, temp sensor, 2 push buttons, 2 LEDs, and an ADC pin pulled out.  

You might consider getting this: http://www.bluegiga.com/BLE112_Bluetooth_low_energy_module and just soldering it onto a standard two layer PCB that has all peripherals on the bottom layer.  BLE112 is essentially a CC2540 with two oscillators, RF parts and decoupling under a shielding can.  Here is one with no RF shielding can.

Leo