Purchase, install, play.
I've used three different sizes on builds and find that the larger disc seem to produce more sound power. As far as tone goes, they all seem to be very similar. One thing I recently found is that the location of the disc inside the box has a major effect on the tone quality. Common advice is to put it under the bridge on the bass side, but I've spent a good deal of time moving the piezo around to see what effect it had. Location does have a major effect on tone. For my Punch cigar box, I found that placing the piezo pretty much in the center of the lid (both vertically and horizontally) produced some very warm tones. I also noticed that holding the box close to my body dampened the sound noticeably. I guess the box bottom adds a lot to the vibrations picked up by the piezo.
Remember, a piezo is more like a microphone than a mag pickup. finding the spot where it picks up the best sound will make a big difference on the final sound of your build.
Another thing to consider is a passive tone control. Some folks haven't had much luck with tone controls on piezos, but there are some designs on this site that do have an effect.
Good luck with your piezo project.
I did an experiment with two identical piezos and a blend potentiometer. One piezo was under the bridge, the other was on the bass side about midway between bridge and neck and about 1-2" away from the strings -toward the player's face, if you can imagine.
Turn the pot one way and you get a nice spanky treble-y sound, turn it the other way and it gets darker and warmer. Not much difference in volume either.
Since then I've switched over to magnetics - far easier to prevent squealing with your amp on 11.
I think the ultimate experiment would be to wire a pile of piezos all around, in parallel, with individual volume knobs on each. Kinda like those funky old 5-pickup Japanese guitars from the 60's.
when you do install them try different placements move them a round tell you like the sound everyone is different. Don't waste your time with some intensive way to install them for good make it easy and simple. Try a cap. across the input jack.
use the search engine sometime it takes awhile but you will find it here.
A capacitor across the leads of the jack will bleed off some of the treble. The challenge is to find the correct value to get the sound you are looking for. I've tried everything from .0022 uF to .022 uF. The .022 really bleeds too much treble, so something less may work better for you.
I suggest that you first find the best location for the piezo and then finalize the tone with a cap.
I think it is called a VariTone switch. Might be interesting to add once you figure out which caps work for you.
Yep , try different thing until your get the best sound the cap. really helps control the pickup as does the way you mount then. That is if your not using a tone pot that's another story.