Differing the pH of the recalculating water in the calcium reactor affects the rate of dissolution primarily, and the only effect having larger medium chunks is that it exposes less surface area per gram of medium. This is only a benefit if the medium itself is relatively soft or porous and you don't want to dissolve it too quickly (so you run the pH up to the 6.7 to 6.8 range). Ideally you would want to have reactor medium that is uniform in size and hardness, sized about the size of coarse aquarium gravel (like the ARM medium) so that for each fill of the reactor, you get a relatively consistent delivery rate of calcium and alk over the duration of each medium fill. Really there is no reason to not do this, just try busting the skeletons up so that they are somewhere in the vicinity of the ARM medium size for the sake of consistent delivery. Aragonite = parrotfish poop = coral skeletons, or mined aragonite which = old coral skeletons.
Ideally for most reactors in the average system (if properly sized), starting in the 6.65 to 6.75 range is going to deliver appropriate amounts of calcium and alkalinity, and following trends in the tank will tell you if you need to bump up the delivery of calcium and alkalinity, either by initially increasing the throughput rate of the effluent, or by dialing up the amount of CO2 by lowering the range for the controller to dissolve more aragonite to meet your system's needs. If the throughput is maxed out, then increasing the acidity of the reactor will dissolve the calcium at a higher rate, resulting in a more concentrated effluent being delivered to the tank. The effect of decreasing the size of the reactor medium is that it will dissolve at a faster rate for a given pH within the reactor. This is true down to a certain point, after which the medium become so small that it blows away with the recirc current or turns into mush if the flow rate is slow. The advantage of using a consistent size of medium is that it will allow you to have a consistent delivery of your hermatypic supplements, so that you should be able to refill the medium chamber in the reactor and seal them back up without having any need for readjusting the settings of the reactor or controller (make sure to clean the effluent control valve with white vinegar!~).
HTH
