All trace sizes, actually everything, should be set on the schematic.
Every trace has a Net Class associated with it. If you haven't been specifying it, they're all part of the Default class which has no width specified, so width defaults to 10mils during the Routing stage.
So, go under Edit->Net Classes, Default-Width-30 mils (or whatever floats your boat).
You can also add say a Power class and make it thicker. You can specify this when making connections. Or you can Edit->Change->Net Classes, select Power, double click on each wire you want to become part of the Power class. This is how you can have thick and thin wires coexisting on the same design.
It is also possible to route the board and change with width of individual trace segments. Generally this is not the right way to do it and it will raise warnings on your Design Rule Check but it's otherwise ok. Sometimes you have to do this to make a thick power trace used for Vdd or Gnd turn into a thin trace over a short trace stub in order to connect to a fine pitch SMD pad without extending over the neighboring pads.