The plate size gets all the attention and the box depth decides whether the job is possible. Almost every "my smart switch does not fit" story is a depth story, not a size story.
Two measurements that are not the same
A mechanical switch is a plate, a rocker and two terminals. Measured from the wall face it might sit 25 mm behind the plaster. A smart switch module is that plus a power supply, a relay or triac bank, a radio board and an antenna keep-out area. Measured the same way it commonly needs 35 to 50 mm, and a six-gang plate is worse, because the power supply and the terminal blocks grow with gang count rather than staying constant.
The box in the wall was specified for what was there when the building was wired. A standard 86 mm round or square box is often 35 mm deep in older work and can be as shallow as 25 mm in partition walls. Put a 45 mm module into a 35 mm box and the plate will not sit flat, the fixing screws will not reach, and forcing it bends the frame so the touch panel reads falsely forever after.
What else has to fit behind the plate
Terminals are the second constraint. A smart module with a neutral has more conductors than the mechanical switch it replaces, and those conductors need bend radius, not just volume. Crushing them to make the plate close is how you get a terminal that warms up and a switch that brownouts under load.
Then there is heat. A sealed box with a power supply running continuously is warmer than the box it replaced, and warmth plus a tight bundle of conductors is exactly the combination that ages insulation fastest. Derating exists because of the box, not because of the module.
The three fixes, in order of how much mess they make
The first is to measure before you buy. Take the plate off, measure from the wall face to the back of the box, and compare against the depth in the datasheet rather than the depth on the listing photograph. If the number is close, assume it will not fit, because the datasheet figure usually excludes the terminals.
The second is a deeper box. There are retrofit boxes designed to be swapped into an existing opening, and on a masonry wall that is a chisel job of maybe twenty minutes. On stud partition it is usually easier than it sounds because the plasterboard is already cut.
The third is a surface-mount or a shallower module. Some ranges publish a reduced-depth variant specifically for retrofit, sometimes at the cost of one gang or of the neutral terminal. Choosing that is better than choosing a module that will be permanently under mechanical stress.
The one that does not work
Trimming the module, filing the box, or 3D printing a spacer are all ways of converting an electrical problem into a fire problem. The clearance behind a mains terminal is not packaging, it is part of the insulation system. If the depth genuinely is not there, the honest answers are a deeper box, a shallower module, or moving the smart function somewhere else in the circuit, such as a DIN-mounted relay in the consumer unit with the wall switch kept as an input.
Where this matters when you are buying
Check the hardware first: smart switches.
Panel standards, protocols and MOQ live on the Xingliao smart home factory site; panel families (86, 118, 147) are on the smart switch board manufacturing page. Sourcing questions go to the factory team.
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