The track size of any pcb is really important particularly when high currents are involved. Such a materia is strictly ruled by a lot of directives, almost always of military origins, not really important for a daily use.
All these directives (for people intereted in) are listed, public and published: here the link
In an hobbystic environment such directives are not really important, what is really necessary is to follow some simple rules. Pcb are sold or realized normally into two different thickness: 35(micro)m and 70(micro)m. According to such a condition we can set the size according to the following table:
width for 35um (mils)
width for 70 um (mils)
But what happens if, for instance, a big track must be connected from the bottom to the top layer? the secret consists in create simple pads to join the two tracks (top and bottom) in multiple points