Just as there's many ways to kick a dwarf, there's several ways to smoosh a screenshot of this strava heat map you so covet.
With regards to the xrandr route, xrandr allows a display size up to 10,000 x 10,000. Rather than doubling the scale, simply keep the scale the same and enlarge the display size. The browser will, or at least should stretch out and the off-screen content will continue to render. Then it's a matter of taking a screenshot of the entire desktop.
A shell script like this might do you well:
xrandr --output HDMI1 --mode 1920x1080 --panning 9000x9000 --scale 1.0x1.0 # allow time for content to download sleep 200 # take your screenshot scrot --quality 100 'screenshot.tiff' # return to original display settings xrandr --output HDMI1 --mode 1920x1080 --panning 1920x1080 --scale 1.0x1.0
General tips: Once the display has enlarged, it's quite hard to navigate and arrange the zoom level etc. It's best to have the zoom level preset before modifying the display. Enlarging the display is resource intensive, so it may take some trial and error to determine the best size before your system spaz's out totally. Also, to improve your chances, it's best to run your browser instance with all addons disabled e.g. firefox --safe-mode