Is it possible to attack Westeros from Qarth, if one sail to the East? How far is it in the book?
No one has ever crossed the Sunset Sea to learn what lies on the other side
Based on JRRM's own pronouncements about the size of the world of Westeros, and assuming that the lands of Essos and Westeros are roughly the width of Europe and Central Asia (around 3000 miles), then the distance between Qarth and Pyke could be anything upwards of 75,000 miles of open ocean.
"Interviewer : Is your world round? I mean if Dany traveled far enough east couldnt she come to the other side of westeros?
GRRM : Yes, the world is round. Might be a little larger than ours, though. I was thinking more like Vance's Big Planet.... but don't hold me to that."
That would certainly explain why, when asked, he was categorical that no-one has ever managed to circumnavigate the world.
To the same scale as your map above, the planet would look something like this, assuming that the northern reaches of Westeros extend up into the poles.