Whew.
The Front L&R - B is so you can use the receiver in another room. My Denon allows me to feed different sources to different rooms. However, the feature uses the amps for the surrounds, so you can get full HT in one room, or stereo in two rooms. I believe some of the newer 7.1 systems allow you to use the LFE amps instead.
Personally, I would use the same speaker for all front L-C-R. You will find that the center is the speaker that gets the most demanded of it. Don't scrimp on center, and try to get them to flow from left to right.
Physical size of rears is not that important, except aesthetically.