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)
bgm1911
@OP,
I hate to be the stuck record, but have you considered Vandersteen?
IMHO, one of the best performance to price speakers out there.
If you can wait, a used pair of Treo's or Quatro's would be under $9K.
Bob
I thought that would be a very easy way to flesh out your budget..........

Now don't make me beg for the amp/preamp and source you have or hope to have....
If your heart is set on Focal Sopra 2's they will work greeat in your room.  I have had smaller speakers in a larger room that you are talking about and they worked fine.  I assume you will be listening to them at 10-15 feet away.  Your bound to get tons of suggestions to consider other speakers.  I would stick to wha you enjoy.  Room treatments will help if your spouse will allow you to but SR HFT's also help.
15x20 is a good listening room. You can make pretty much anything work in there.  Sopra 2 Sopra 3 both would be fine. 
I have a Thiel CS3.7 in a 11 x 12 x 9 (plus extra room from closet). See my Virtual System photos. The room is treated with GIK acoustic panels and the digital streaming source is treated with DSP that was setup remotely by a super talented audio engineer. The sound is perfect and I even sit off center.

I have some links on some of my old posts that go into detail of how this was accomplished. Send me a DM if you want more details from me.