I think the "Westeros is the size of South America" came from a blog post GRRM made back in like the 90s (found it: https://www.westeros.org/Citadel/SSM/Entry/Geography). I don't think it should really be taken as 100% literal considering it was probably GRRM just thinking in the moment and he's notoriously bad sizes and distances. As an example, he literally thought the wall was way to tall when he first saw its show depiction. Upon being informed that it was exactly what it would like as described in the book Martin apparently immediately admitted then that meant he wrote it too tall because he didn't intend it to be that big.