I think the Fluff section of the forum suits this topic better. @Killer Angel could you transfer this question there? Grrr, !mrahil
To get to your question... are you looking for longest length and widest width (bounding box)? Grrr, !mrahil
I have taken a look through the battletomes and found this map in the 7th edition(?) book (the red soft cover one) With the bottom left distance indication you could work it out Grrr, !mrahil
Done. Regarding the question... if it can help, "South America has an area of 17,840,000 square kilometers (6,890,000 sq mi)."
Perhaps I am incorrect, but I have long operated under the assumption that the Warhammer Fantasy world has twice the surface area of real world Earth. Thus, Lustria would have a surface area of roughly 35 million square kilometers.