You may be surprised to hear that up to 80% of a movies soundtrack is carried by the Center speaker. If you don't believe me, disconnect the Center speaker from the amp end and watch a film
Ideally all 3 speakers up front should be of the same size and type, but in the ideal world this would never happen. Center speakers can come in all shapes and sizes. What make and model are you actually looking at ?
100Hz - 20Khz sound about right if you are using a Sub to take everything below 100hz anyway. Just make sure the crossover on the amp is set up correctly.
Its mostly a Question of what you want to achieve from your Speakers, How you intend to drive them ... What sort of Receiver/Amplifier and how you intend to Feed them .. CD , DVD Blue Ray .. ??? AND how much do you intend to pay for all of this Stuff
Its Not How Big It Is But How You Use It that matters.
Oh and the Sound that goes through the center speaker is the sound that Should Go through the Center and that depends on the illusion that you intend to create in your viewing room, which in turn is dependent upon the size of your viewing screen.
Keep in mind that the crossover for the Sub, is really about the Sub relative to the Fronts. The Center is a completely separate thing.
You would logically expect reasonably full response across the vocal range which is roughly 82hz to 880hz. But we would want a fair amount of harmonics to flesh the sound out. But I don't imagine much in the way of bass below about 100hz and certainly virtually nothing below 80hz. Anything in that range is coming from the fronts and Sub.
So, I think a 100hz low end is acceptable especially when combined with other matching components from the same manufacturer. Acceptable, but not necessarily ideal. It would seem more ideal for the Center to be a moderately full range speaker, as most of the speakers listed above seem to be.
As far as how big Centers are, how big did you expect them to be? They are speakers like any other. Certainly you can get tiny Centers if you are willing to accept tiny performance. But if you want big full clear sound then within limits you need big speakers.
I can tell the difference in lower bass between a 85hz and 44hz centre speaker. And it's not just lower bass either, it's power handling. A speaker with 2 bass drivers and a mid range is going to be able to play louder and cleaner than a single 1 mid 1 bass driver.
The centre does more than just voice, primary effects come from the centre also. Watch Conan the Barbarian and James Earl Jones's voice is far more deeper with the Model 200.