I usually just target the font size of the icon. It looks like the icons are usually 4em by default, so I just adjust from there.
Half size: (2 / 4)
.tile .icon svg { font-size: 2em; }
25% bigger (5 / 4):
.tile .icon svg { font-size: 5em; }
I was trying to figure out why you had all the other stuff with positioning in your example since the icons are already centered, but I guess once you make the icons so big that they go out of their bounding box, the extra centering can help force them back to center.