The flags are svg, meaning that you could use any size
Geo Flags it's a simple addon for the Geotargeting Pro plugin that will let you display the current user country flag or any other flag that you want by using a simple shortcode like this:
[geo-flag squared="false" size="100px"]
All images are SVG and can scale as much as you want without loosing quality.
FAQ
What sizes are available?
What shapes are available?
Geo Flags can be added square or rectangle shape
Where can I learn more about geo flags?
You can read the docs page for more options for the shortcode.