If you're trying to compare country sizes, figure out travel times for your book (or vacation), or explain to visitors why they can't pop from NYC to LA for lunch allow me to introduce you to the handy dandy tool that is TrueSize.com.
If you're trying to find where I live on the map, neither South Korea nor Republic of Korea work. Use plain Korea and you'll get the country that has Seoul in it.
If you want to blow someone's mind, show them the USA compared to Africa or Alaska. Both tend to throw people for a loop.
Egypt is a fun one for Americans. Egypt is as large as the four corners area (UT, CO, AZ, NM) combined.
@LianaBrooks Reminds me of this book, excellent but now very dated. Besides sizes it shows how the world looks from the vantage point of various countries.
A Strategic Atlas: Comparative Geopolitics of the World's Powers https://a.co/d/6njDBc6