how to calculate resistor needed

I would start by calculating the current needed for the bulb -

P = V(A)

Now that you know the voltage and current the resistance of the bulb can be determined using Ohm's Law -

You could then pull up a voltage divider calculator like this one: http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm

Plug in your battery voltage (6), your R2 (30) and your voltage out (2.5) and calculate.

The end result is a 1/2 watt resistor of 42 ohms.  If you look at all the values given, though, you'll see a slight problem - the current through the bulb will only be 0.083A - under half of what the rated current is.  The bulb may or may not work at this current - you'll have to experament with it.  The problem with voltage dividers is their limited current output.

To incrase the output you could go with an adjustable voltage regulator and set it to 2.5vdc.  At that point, your current wouldn't be limited and you would give the bulb max power at all times.

If a 20 ohm resistor made it glow dimly then a 40 ohm is going to make it glow even less.  The heat associated with the resistor would be from the fact that the current going through the resistor would be .120mA at 3.6vdc which would be just under 1/2 watt.  You would need a 1 watt resistor to prevent it from getting hot.

An adjustable voltage regulator would be something like the LM317 - it uses a few components to select the desired voltage output.

Another solution may be retrofitting it with an LED instead of an incandescent bulb.  LEDs are bit easier to work with when it comes to weird voltages as they are discreet components (instead of resistive).  There are a few tricks you can use to make them work.