what is a microstrip
Microstrip is build in a following way. for example you have 2-sided pcb where the bottom layer is left as a ground plane. Then on the top side you have a copper trace. Between these bottom and top layer is of course the pcb substrate. Now, changing the width of the copper trace affest on the characteristic impedance of this microstip. So you have to do some math in calculating the trace width, since it is affected also by the dielectric constant of the substrate and the thickness of the substrate. And if we are really accurate, then we should also take into account the thickness of the top copper trace. Take a look some microwave engineering book (li Pozar) and find out those equations to calculate microstrio trace width. Or you can find ready made calculator from the web... which suits you better
Hope this heps