You can call the getCoverArt view and specify an arbitrarily large size parameter, which will generate a large image and tie up server resourced for up to a minute. The only upper limit seems to occur when the album art thread crashes from the excess memory usage.
Example call:
/rest/getCoverArt.view?v=1.6.0&c=subweb&f=json&size=9001&id=<album art id>