Increasing a non-retina screenshot to 2X size will not be any different than showing the original.
Instead, alter your browser settings to emulate a retina display:
Firefox: Go to about:config, and change layout.css.devPixelsPerPx from -1 to 2
Chrome:
- Zoom to 200%
- At the bottom of the JS console (Option-Command-J), click on “Emulation”
- Check the “Emulate screen” box
- Set “Device pixel ratio” to 2
- Make sure “Emulate viewport” is unchecked (otherwise the content gets resized)
- Uncheck “Shrink to fit” (it doesn’t seem to do anything useful)
- Uncheck “Enable text autosizing”
Source: https://wincent.com/wiki/Taking_Retina_screenshots_on_a_non-Retina_display