Argh, just did some math from that authors own page and found some numbers which are inconsistent (god damn you Martin). The following calculations will be going off of the initial book 1 trip from Winterfell to Kings Landing which took approximately 3 months (90–100 days).
That party was a Royal Party with a wheelhouse, which shouldn’t be travelling faster than 10 miles per day. Going off of that rate in order to reach Kings Landing in 3 months like they do in the book the distance between Winterfell and Kings Landing at the absolute maximum could not be more than 1000 miles (I’m working roughly here rather than exact). But wait that can’t be right! The website I originally linked to lists the distance between Winterfell and Kingslanding as around 2000 miles! Oh dear.
Taking that absurdity into account I would propose that I half the size of Westoros. That way Westoros would be about 2k miles from the wall to the southern most tip of Dorne.