First of all, it isn't the physical size of the speaker that you are interested in; it is the sound output. Not all speakers the same size have the same output, so buying by size is silly. (Often, a larger speaker will play louder, but this is not always the case, so do not simply buy by size.) What you want is a speaker that will play as loud as you will ever want to hear, as far away as you will ever be, without excessive distortion, in the particular space you have. Since individual rooms are so different from each other in terms of sound absorption and sound reflection, it is a bit difficult to say what you need without trying things out. All else being equal, a larger room will require higher output for the same volume, but all else is rarely equal, and it also depends on how far away one will sit; it does not matter how loud it is at the far end of the room if you are not there. Also, I do not know if you want to listen at deafening sound levels, or at some more sane level. What you want is difficult for me to know, but is essential to know if one is to say precisely what you need in order to get what you want. So the first thing to remember is this: Buy from someone with a good return policy. That way, you can try it out in your room, and if it is not satisfactory, you can return it for a full refund.
With speakers, there are ratings for output. You will typically see things like 90 dB spl @ 1 watt @ 1 meter. You will also typically see a maximum power rating, such as, 100 watts. From this, you can guess that the maximum output possible with the speaker will be about 110 dB @ 1 meter (because with 10 times the power, you get a 10 dB increase, so at 10 watts, you would have 100 dB, and at 100 watts, you would have 110 dB; with a doubling of power, you get a 3 dB increase in volume). However, there are a couple of other things to remember, the first being that you will likely be more than 1 meter away from the speakers, and the dropoff will depend on room acoustics. A rule of thumb for a "typical" room is a loss of 3 dB for sitting another meter away, though, again, the particular room will make this unreliable. Also, maximum power ratings are not standardized, so there is no way of knowing if the speaker will acceptably be able to reproduce the 110 dB in the first place. It should be taken merely as a rough guide, not as anything absolute. However, within the same brand, since the manufacturer probably uses the same standards throughout their lineup (though they may not), you can generally conclude that the one rated for higher power can handle more power than the one rated for less.
All of this, of course, brings me to another point: The sound level that is possible in your room is also dependent upon the power output of your amplifier (receiver).
Another thing to consider when buying speakers is the frequencies that you want them to reproduce. Ideally, all speakers would reproduce the entire frequency range. That is expensive and difficult, so people typically use subwoofers for the deep bass. Now, this works well for very low frequencies, as you cannot tell what direction they are coming from by sound alone. However, as one gets to upper bass, one's ability to discern the direction it is coming from becomes greater and, of course, you want the sound from each channel to sound like it is coming from the appropriate place. So, you want all of your satellite speakers to go low enough. How low is low enough? Well, it depends, as one usually is not listening to test tones, other tones can mask where something seems to be coming from, and your ability to discern direction doesn't go instantly from nonexistent to ease and perfection. The THX standard is 80 Hz for the low end of the satellite speakers, and I recommend sticking with that for good sound. Typically, this means that one must have a 5" (or larger) woofer in each satellite speaker, though it is not the size that you really care about, it is actual performance, so buy based on performance. (Still, you are not likely to find too many speakers that can usably go below 80 Hz that are smaller than 5".) This applies no matter how small the room is. Obviously, there are many satellite speaker systems that do not reproduce 80 Hz adequately, and these are systems where you may on occasion notice that a sound is not coming from the right spot. Whether this will bother you or not is difficult for me to know, but, as I said above, I recommend sticking with satellites that can go down to 80 Hz or (preferably) lower.
And, of course, it is always best to err on the side of something that can do more than you need, assuming that you have the room for it and the money for it.
If you can try out the speakers in a room that is roughly comparable with your room before you buy, that can be helpful, but, of course, since individual rooms have different acoustics from each other, it will be no guarantee that it will work satisfactorily in your room.
