Mostly none, the size of the crystal determines the 'strength' of the signal it generates, and there is some capacitance value in the disk that is a tiny fraction of the value of 0.01uf with bigger disks having a tiny bit greater fraction than a smaller one.
If a rod piezo works fine and its much less crystal than a disk, then a 20 should work as well as a 27.
Mostly, depending upon your amp rig and your ear, its probably an unnoticeable difference, maybe tiny itty bitty difference if you have a sensitive ear and do a side-by-side with identical instruments...
you could always try 2 of the 20 mm disks. area of 2 20mm disks is only slightly bigger than the 27 mm disk..a bit of experimenting might be in order. see what sounds best to you?