Ok. Let's see if I remember this correctly!
The length of an "full length antenna" is determined by the wavelength of the frequency that you will use it for.
To calculate the wavelength you do something like this:
Speed Of Light = 299,792,458 meters per second
Speed Of Light/frequency = wavelength
299,792,458/2.4GHz = 0.125 meters = 4.9 inch
So the optimal length for a 2.4GHz antenna should be 4.9 inches or 12.5cm.
You can also use half length and quarter length antennas. (3.95" or 1.97")
It was a long while since I did this kind of calculations but I think that they are correct!
/Niklas