Yes, you can COMPUTE statistics and then use EXPLAIN PLAN to get the cost of the query for different sort_area_size settings. However, the only differences that you'll notice are the effect of avoiding a disk sort altogether if your sort_area_size is big enough, and the effect of needing multiple merge phases if your sort_area_size is very small. Otherwise, the effect of sort_area_size on an individual query is almost nil.

Is it possible to play what-if with sort_area_size for a given query performence? I mean, other conditions being equal what is the effect of different settings of sort_area_size on query performence?