My Room Size for Speaker Consideration

As I look at possible speaker sizes for my new 2-channel, I'm wondering how I should look at my room dimensions.  The family room is 15' x 20'.  On the 15' length (where I sit), there is a knee wall that opens into the kitchen, which is another 10' x 20':

At it's smallest, it's 300 sq ft
At largest, it's 500. sq ft.

To better match the speaker size, which room size should I consider? 

(I'm dreaming of the Focal Sopra 2, and their website is kind to recommend room dimensions)