To change how sensitive an aircraft is to particular control inputs, adjust the part properties from the right click menu in the SPH. Use a combination of two methods:
Set the maximum movement on control surfaces that are only used for roll (ie your ailerons in this case) via the Authority Limiter. This is the green bar at the bottom of the pane in an unmodded KSP.
Adjust which control surfaces respond to roll input, reducing the amount of torque being applied. In your case, you could switch Roll to "Inactive" on your tail fin and possibly your rear winglet elevators too.
Each method has it's advantages and disadvantages. Consider changing one thing at a time and taking test flights between each so as to not adversely affect other aspects of control by overdoing things here.
On a side note, try seeing how the aircraft handles without SAS turned on to get a better feel for what's going on.