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

Glad to hear it is working out for you. As long as you are driving the 570 with ample power you shouldn't have any problems at higher levels. I use a bryston 7bst and drive it quite hard and don't have any problems what so ever.
Keep us posted.

"A center like the 570 I would crossover somewhere between 50 and 60 hz" (Dedwards).

Besides this center speaker being a typical dynamically low-efficiency design (Reviews measure this speaker at 86db efficiency/about 88 room, and is 5 ohms minimum/under 4 ohms nominal), the reviews and Paradims website list the frequency response limits on this center channel at 60hz+/-
3db, and 65hz +/- 2db! It's basically flat down to 83hz!!
So, if you plan on crossing this speaker at 60-50 hz, you can prepare for a very very weak bass response at the crossover point!!! This is a typical fauxpa by most audio-enthusiests, and should be avoided.
Not only that, but the standard inneficiency of a passive speaker, MANDATES that you get the very demanding bass info to a powered ACTIVE WOOFER, for best results, and dynamic transparancy/impact.
People can argue all they want that their full range audiophile speakers sound faster and tighter in the bass playinf "full range". But the problem lies in the "in-efficiency" of the design! When you play DD/DTS material, that's mixed at 5x's as more demanding dynamically than any recorded music material, and things fall appart real quick with this line of thinking! You get flat sound, dynamically squashed playback, limited dynamics, clipping, and blown drivers...as the amps can't supply the current and control to the drivers through the passive networks! This is standard affair for typical passive hi-fi speaker setups. That's the way it is. If all speakers were active models, with bigger woofers, that wouldn't be the case so much. Heck, even active commercial cinema setup's, with 118db horn designs, still cross over at 80hz to big 15/18" bass woofers!!!! There's a reason THX does this, cause it works!
Still, everything in a balance.
Based on those speakers he's using, and knowing he didn't have a sub right now, I'd be crossing that center over at 80hz, every time, and sending the bass to the mains, yes!
Everything else is going to sucketh. Sorry to inform you.

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.