Size of the exported image depends on the chart size(width and height of the chart). Hence by changing the width & the height of the chart before exporting it and resetting it back should work fine in your case. Please take a look at this JSFiddle.
We don’t want to increase the height and width of image we want to improve the image quality(resolution). Just check on zoom in event the image becomes quiet blur..
The image quality can be improved further by editing the source code of CanvasJS. You can purchase a license from our license page to get access to the source code. Once you have purchased the license please let us know and we will help you out with a solution.