"IT’S BIGGER ON THE INSIDE. " (Sorry, couldn’t resist. )
The point however is that while zwave antennas are pretty big, A light switch device is still designed to fit into a one-gang switch box. There’s usually enough room to just swap it out for the existing switch unless for some reason it was a really really narrow switch box which would be unusual.
So if the switchbox is of standard dimensions, a typical Z wave or zigbee light switch will fit in it.
TWO OUTLETS, BUT ONLY ROOM FOR ONE ANTENNA
The size issue usually comes when you try to fit two different antennas into a standard single gang space. This is why a zwave-controlled outlet with two places to plug things in typically only has zwave control for one of those two and the other is a standard nonnetworked receptacle. But the outside dimensions will be exactly the same as the outlet that’s being replaced.
There is a new zwave standard, Z wave plus, which does have somewhat smaller antennas and we are starting to see more devices coming out using zwave plus. So we may start to see more options over the next year or so for outlets.
But the outer dimensions of all the devices are still the same standard size.
As Ray says read the dimensions carefully before buying anything but I don’t think you’ll run into that particular problem.
THE MOST COMMON ISSUE: WHERE’S THE NEUTRAL?
The most common issue people run into with US houses built between 1940 and 2000 is that they often do not have a neutral wire at the light switch. Most networked switches will need a neutral because they have to be powered on even when they act like they’re off, so that they can still hear the next “on” command.
However, there will be a neutral somewhere it just may not be at the switchbox itself. For example, almost all outlets will have a neutral. And if there isn’t a neutral at the light switch, there should be one at the light fixture.
So for older US houses an electrician can usually “fish up” a neutral from somewhere else in the wall. That allows the Z wave or zigbee switch to be installed.
HOW BIG IS “MICRO”?
The other issue you run into is if you want to keep your existing switch hardware, but put a micro relay inside the switchbox to give it network capability. Basically turning a dumb switch into a smart switch. These devices do still require a neutral for power. But they also have to physically fit inside the switchbox. This is where the question of narrow versus deep comes in. A standard switch box will typically be deep enough to easily fit the micro inside. But there are some narrow switch boxes Which are to code but may not have enough room for the micro.
So if you think you will want to use micro relays with the existing switch hardware, then you need to have the electrician check each switch box to make sure there will be room.
But if you just want to swap out a smart switch for a dumb switch, the outer dimensions should usually be the same.