Hello,
Changing stroke-width in the code, or from Properties Panel should change the stroke size for the canvas shape.
Please make sure you apply the color to the stroke.
If you still don't see the change, please let us know more details.
The change of the stroke size will affect all keyframes. Animating the canvas for it's color, stroke and shape is not currently supported in GWD.
Thanks,
Mariko - Google Web Designer