Html2Canvas Resize

Based on Mikkos hint the following woked for me

window.html2canvas([$('body')[0]], {
              onrendered: function(canvas) {
                var extra_canvas = document.createElement("canvas");
                extra_canvas.setAttribute('width',70);
                extra_canvas.setAttribute('height',70);
                var ctx = extra_canvas.getContext('2d');
                ctx.drawImage(canvas,0,0,canvas.width, canvas.height,0,0,70,70);
                var dataURL = extra_canvas.toDataURL();
                var img = $(document.createElement('img'));
                img.attr('src', dataURL);
                // insert the thumbnail at the top of the page
                $('body').prepend(img);
              },
            });

You can create additional new <canvas> with thumbnail dimensions and use drawImage() to scale it down on this new <canvas>.

drawImage() can read <canvas> as image source and you may set target width and height.

https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Canvas_tutorial/Using_images