How do I select object using its name?

Suppose I want to save an object's width in a variable,so I will save car.width in that variable (object name is car)

But what if I want to do same thing using just "name" of the object? Something like "object_name".width

You can do this in a script:

runtime.objects["ObjName"].getFirstInstance().width

In events the only way to do this is to add all objects into a family and pick family instance by ObjectTypeName:

System Pick Family by evaluate Family.ObjectTypeName="ObjName"

After that you can use Family.width