Room size for large speakers?

I auditioned speakers that are 46" tall that sounded great in the room they were in. I got a great deal on a larger model of the same brand which is 49.5" tall so I bought them without listening. My room is about 25x15x7. It's hard to tell what a "large room" is considering that is a relative term. I'm hoping they don't overpower the room.

Depends on the speaker. Room ratios are often as important as volume. Line arrays and planers have less ceiling interaction than dynamic speakers--so this can affect what ceiling height might be appropriate. Construction techniques can also affect overall acosutical volume--for example a stiff wall will make the room acoustically hold energy and thus reduce it's effective volume, where as a flimsy wall will leak energy. How the room is engineered also has a huge effect.

I realize I haven't really answered your question--but that's because there isn't one ideal room size. I'm glad you asked the question though because we get this question regularly--or similar types of very open ended questions like "what's the best room dimensions?" without any other qualifiers. Again--those types of things are dependent on many additional variables.

My experience is that there is a dranatic improvement between "bedroom" sized rooms (12 by 14 feet) and rooms in the 20 foot range. After about 30 foot size you still get improvement but diminishing returns. Much greater than 50 feet becomes a problem becuase of large delayed reverb IMHO.

Of course as Rives correctly implies... a good treated small room may sound better than a bad untreated mid sized room...