microstrip line design

Hi,
I want to know how to decide the electrical length in the design of microstrip line.suppose I want to design a 50 ohm line with a substrate of dielectric constant=6.45 ,H=5mils ,T=0.35 mils,tan D=0.0027 which is supposed to work from 1.5GHz to 2.5GHz,what I have to enter in microstripline calculators in the place of electrical length and how is the electrical length decided?

The line length is whatever you need for your design. The calculator does not need this for analyzing the line impedance.

Then, why does the calculator ask for a length?
Due to effective epsilon, the wavelength in your microstrip is smaller than the wavelength in air. The microstrip calculators help you to find out the effective wavelength for your microstrip line. You enter the required length in ° phase and a frequency, and the calculator tells you the corresponding physical length. This is only for information, has NO effect on the calculation of with/line impedance.