How big should center spkr be to set to "large"?

HOWEVER, another problem here is that many people will end up "unknowingly" placing their speakers in the room, in conjuction with their seating locations, where the bass response is going to be all over the place. They often end up putting speakers where there's a hole somewhere between 60-100 hz. Then, when they put their crossover in this reigion, and it sounds like there's a "hole in the sound"/at the crossover, THEN THEY LOWER THE CROSSOVER POINT! but the problem was in the setup all along...user error.
This is one of many many reasons audio-enthusiests read reviews or hear speakers and gear at some audio store or show, then they buy some stuff off the net, hook it up in their home, without knowing what they're doing, and get bad results so often. They then conclude that the speakers aren't so good, or the sub's not as good as something else, etc.
When it comes to home theater/audio room setup, it's all about THE TEAM! Balance...

Thanks Ozz,

I am driving all 5 channels with a Sherbourn 5/1500A, which is a 5 mono channel design @ 200 wpc. Weighs about 80 pounds.

I have the Legacys bi-wired, and they sound pretty good.

Guys, I understand a good sub will make things better, but that is not in the cards right now.

Also, we don't really crank it up loud, so I don't get too much of the sound falling apart problem.

"Also, we don't really crank it up loud, so I don't get too much of the sound falling apart problem"

This is not the problem for you here! Even if you listen at a "flee's level", well bellow the average listener, the problem lies in the dynamic ability and accuracy of the sound! Put your center speaker in a place where it's not coupling well at the crossover, or cross the speaker over where the speaker is "quickly" rolling off, and you'll have lower fidelity, even weak sound from the speaker (and thus the system)!
It's not necessarily a matter of whether or not you like to crank your system. If the system is going to have any chance to have any sense of power, dynamics, impact, and accuracy of response to the sound, you need to consider these things. Otherwise, you cross your fingers and hope for the best. Often times, it doesn't turn out so well, being there's so many speakers, so many settings, crossovers, phase issues, acoustical issues, fine-tuning adjustements, etc, that it all gets washed over...pulling the system down the toilet, sonically.
If I were you', I'd do this test:

Run the center speaker full range (mono, summed stereo), with a test CD that has test-tones at all the lower frequencies. Play them through the critical 63hz-100hz reigion, and see what kind of response you're getting from your center! If you're getting weak response (in relation to the rest of the frequencies, and at 1khz) at the 80hz point, or where you cross over, you've got a problem! That's where the critical bass reigion lies. You should be getting solid response down where you're crossing your speaker over, in this case, to your mains. See what you get.

Thanks,

I don't know if my receiver(Sony DA4ES) offers a choice of frequencies to "cross over" the center channel. I only saw the choices of large, small, or none for the center speaker. Unfortunately, I am not too familiar with the cross over concept and procedures.

"Unfortunately, I am not too familiar with the cross over concept and procedures" (Mtrot)

Ah, you're not alone! Most even seasoned audiophiles, who've tried and followed all kinds of gear over the years, don't know any of this either!

No worries. Try this: Simply play testones through your center speaker, with your "small" setting, and measure your levels compared to 1khz. If you measure like 6, 7, or 8+ db down at 80hz to 100hz, you seriously need to check your center speaker placement, As this is where your crossover is going to be set! Many people cross their bookshelve speakers and center channels where there's "holes" in these critical crossover reigions, and things don't end up so good. If this is the case for you, from your listening possitions (where you measure from), you need to move the speaker around till you get flat response (+/-5db, or better), and especially, no holes/dips in the response above and at the crossover point. Try this and see what I mean. If you can get solid response, you'll have solid dynamic "accurate" sound, fundamentally.
Otherwise, you pay someone, that's all your options