I know this is a bit late, but I was actually looking around and
this article suggests it's about 8% larger than Earth, based on some speculation regarding Westeros' geological history and climate zones. Unfortunately I couldn't find a Worlda sized-map of Westeros so I didn't feel comfortable putting an 8% larger Worlda of it together, I did find
this map but this is a bigger map than a standard Worlda and Westeros is bigger than South America on the standard Worlda. If anyone has an actual Worlda sized map of ASOIAF let me know and I can quickly throw it together.
However, I did feel that the size difference wasn't prohibitive enough to stop me from making the 'Big World' version, which is three times larger than Earth, and I thought that'd be good fun. The map isn't very up to date, but it'll do for our purposes. As you can see this version of the world is
massive, and I imagine you could have endless alien cultures throughout. I know that Sothoryos is at least as long as Essos is wide, and I saw some interesting suggestions that if the planet's climates scaled at this size the equator might just be too hot for habitation whatsoever, at least by Humans.