How large of a speaker do I need

It all depends on how loud you want to listen, how much bass you expect and what kind of music you play. No simple answer.

Budget? Decor considerations?

As to your question about bass, if anything, the open end of the room will help longer (i.e., lower bass) soundwaves form. You are going to have a serious resonant node because of the 15' square dimension, however. That's going to be your biggest concern.

15'x15' is not particularly big. You could easily fill that with 50-100 watts/channel and speakers of 90dB sensitivity or above. Something like an Onkyo A-9555 or Cambridge 640A integrated and a pair of PSB Image T45 or T55s. Or if you have more money to spend, a Nuforce or Bryston integrated and a pair of Totem Hawks or Sttaffs, Paradigm Reference Signature S6, or similar. Paradigm's Monitor 11's sensitivity would be like tripling your power (claimed 97dB sensitivity in-room).

FYI-My present system consists of McCormack ALD w/phono, McCormack DNA .5 (100 watts), Sota Comet TT, NAD cd player, NAD tuner and MIT cables. Presently I am using klipsch kg4 speakers. I like to listen at moderate to loud music, various forms of rock/pop music. The room is not 15 x 15 square, but is more like 14 x 16-slight rectangle. I am considering used Alon 2, Vandersteen 2c, Thiel 2.2 or some Totems. I am leaning torward the Alons or Ttoems becuase I really love the holographic sound stage they produce. Thanks for you input.