It's impossible to give a precise area for the "Southern Great Plain" without more context. "Great Plain" could refer to several different geographical regions.

Here are some possibilities:

* The Great Plains of North America: If you're referring to the Great Plains in North America, there is no universally accepted definition of what constitutes the "Southern Great Plains". Some might consider it to be the southern portion of the Great Plains region, which extends from central Texas to the Dakotas. However, there is no definitive boundary.

* The Great Plains of Eurasia: The Great Plains of Eurasia are a vast expanse that encompasses parts of Russia, Kazakhstan, and Ukraine. Again, there is no universally accepted definition of a "Southern Great Plains" within this region.

To get a more precise answer, please provide more information about what you are referring to.