in Constraint Editor CES set the width for typical, minimum and expanded width.
Then during interactive routing you can, with the right mouse click, choose between the three sizes.
If you need you can define different rule areas for different situation and different sets of trace sizes. when the trace enters a room with different rule area settings, the trace will automatically choose the right size.
Hope this helps
There is also a keyin command (CW) that allows you to change the width on the fly even during plow mode.
It does not rely on trace width defined in Constraint system and allow some flexibility.
i.e. cw 0.1 will change selected trace(s) to 0.1 units. If you are in plow mode the next segment(s) you are going to trace will use the 0.1 value just typed.