Does bluetooth antenna length so critical? What will happen if I increase it to 15cm?

Hi,
What will happen if I am connecting a 15cm wire to bluetooth antenna on the pcb? Will it reduce the antenna gain?

Also if I am taking coaxial wire of length 'X' from the external antenna connection point on the bluetooth module and I am connecting a normal single stranded wire of length 'Y' at the other end (center) of the coaxial cable(2.4GHz rf cable), then what will be the effective antenna length? It will be 'Y' only or the coaxial cable (2.4GHz rf cable) will have any influence in the effective length?

Also is there any effect on increasing the thickness of antenna wire?

If you wish to learn what happens, please read a good textbook on antennas.
If you wish to know what happens, simply try it. Varying antenna length varies its impedance, gain and radiation pattern. Do read a good book!