Though the other answers already said that you don't have to care about impedance matching in your case, the time will come where you may need it. More often, you may want to specify wider tracks for higher currents or more clearance for high voltage.
Eagle supports this, and it's called net class.
In the schematic or layout editor, goto menu
Edit > Net classes... and define sets of wire width, via drill size and clearance.
Rightclick > Properties... on a wire / net you can assign one of the defined net classes.
This way, the auto router automatically uses these different settings for different nets. Unfortunately, there seems to be no difference when routing the board by hand. At least, the DRC throws an error when you used the wrong size somewhere. When you pour a ground plane, the clearance is taken into account.
Here is an example. The first wire is of the standard net class, the second from a net class with 36mil trace width and the third from a net class with 36mil clearance: