43. Laos has a land area comparable to "that" of Great Britain but a population of only four million people, many of whom are members of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.
Is that grammartically reasonable to replace "that" for "the size"? (I know that "that=area")
How about changing the repeated Noun in the comparsion by another generalized Noun rather than "that"?
I think it is better for you to refer detailed explaination of this one on ChaseDream Prep2008. Here is the link http://forum.chasedream.com/GMAT_SC/thread-484446-1-1.html.
I remembered the author answered your question by listing the usage of that, one, those, and ones as pronoun in different situations.