Are you talking about the switches themselves (ie, the white plastic part) or the metal guard that goes around?
I’ve always been able to get my switches to fit. I have plaster walls in my house. Sometimes the plaster “spilled over” in the outlet/switch area a little bit which was never a problem with the old switches, but the Zwave stuff is wider/deeper of course so I’ve needed to “chisel” out this extra plaster sometimes to make sure it fit. But they’ve always fit inside the box in the wall.
Now, that metal guard is another story. Those are designed as if they were only going into a single box. If you’re putting multiple together in a double or triple box you need to break off the metal over hang on the sides that are next to each other.
If this switch was going on the left of a double box, you’d need to break off those three tabs on the right of the switch. They’re scored so just grab 'em with a pliers and bend back and forth a few times to snap them off.
Likewise, if this switch was going on the right of a double box, you’d break off the tabs on the left. If it was going in the center of a triple box, break off the tabs on both sides.
I’ve seen some switches with a full length line (rather than tabs) that needed to be snapped off. Again, though, it was scored to make it relatively easy.