To resize from the left end you will have to change both the width and the X position of the Canvas. This is my initial thought.
But why would you want to resize from the left side. You can go with right side resizing and allow the user to drag the canvas whereever they want, which indirectly fulfills your left side resizing
This is just my thought. If it is real business requirement, then you got to do it :)
Karan