How to set the width of all the traces together to the desired value before using autoroute option in Autodesk Eagle?
This is done with classes.  Do HELP CLASS.  There is also a global minimum trace width specified in the DRC settings.

Each net has a class.  Classes have properties like the minimum trace width and clearance from other traces.

You'd typically use this mechanism by creating a class for the high voltage AC input lines, the high current main power supply, ordinary signals, etc.  It's common to have 3 to 6 classes in a design.