Size of Midrange Drivers

Depends how big is your room and how loud you want to be.
8" or higher midrange driver can push significant amount of air and on the midrange frequencies it's quite a lot to fill with sound small auditorium sized rooms.

Hi Dweller, A few answers....
The obvious, we use a mid range to play mids... smaller than a 4 to 5 inch will normally not play low enough to cover the entire vocal range... many drivers above 5 inch have cone break up or frequency response that will alter the upper vocal range... In an ideal world, a midrange will cover the entire vocal range... No crossover in the vocal region, smooth to no phase shifts and smooth accurate frequency response are most easily found in that 5 inch range.
I have heard very good 15 inch 2 way speakers, this required a 15 that went out quite well and a high frequency driver that would go down and do a good job covering the entire frequency range. So it can be done with many sizes... there is something to say for sensitivity and the dynamics that high sensitivity speakers produce. You seldom find 5 inch mids that are very sensitive 85 to 90 is typical... But there are larger coned drivers that do a good job and it is certainly possible to do a good job with a crossover in the vocal region. As far as Super materials... you can't trump the laws of physics, Mass still cost sensitivity, too low mass still cost low frequency extension and effects box size needed... the super materials have helped with a more detail in our music and lower distortion without a cost to frequency extension. MTM or D'Appolito type designs help give more base output with a small footprint 2 - 8 inch woofers will move as much air as a single 11 inch driver, 2 - 6 inch do move about the same amount of air as a 9 inch woofer, yet still keep the midrange of an 6 to 8 inch... But MTM's do require a very low crossover point from the tweeter to keep from having lobing issues, dispersion issues, smearing of midrange and top notch imaging... I'm sure I'm raising more questions than answering, but about the best that I can do to address your questions, I hope this helps, Tim

08-21-14: Timlub
Hi Dweller, A few answers....
The obvious, we use a mid range to play mids... smaller than a 4 to 5 inch will normally not play low enough to cover the entire vocal range... many drivers above 5 inch have cone break up or frequency response that will alter the upper vocal range... In an ideal world, a midrange will cover the entire vocal range... No crossover in the vocal region, smooth to no phase shifts and smooth accurate frequency response are most easily found in that 5 inch range.

good to read that what I wrote in my post above agrees with this part of your post....

Walsh drivers are a uniquely effective design for mid-range in particular. Modern OHM CLS design Walsh driver covers everything up to 7khz which covers most of what one hears. More exotic versions, old and new can go even higher. Walsh driver size of these varies from CD size to 10" or perhaps more. ALl cover midrange similarly. Larer drivers are needed for low frequencies in larger rooms.

Revel Salon use about 4.5 inch drivers for good reason. Look into driver size, dynamics and dispersion.