In the US, a single-gang electrical box has an interior size of about 2 7/8 inches height and 2 1/8 inches width. That size can vary a little bit because the thing that's actually standard is the space between the top and bottom screw holes for the face plate, which is not included in the size I just mentioned.

I may design a custom PCB so I don't have things loose inside the box, and I think that size rectangle will give me plenty of space for the layout. If it's too small, I'll either have to go to a double-gang box or maybe sacrifice some features to recover board real estate.

That's all for the main switch box. The remote switch boxes won't have much in them, so they'll easily fit in a single-gang box.

I have some face plates that have a single round hole for an RG-59 or RG-6 connector for cable TV. I'm hoping that the size is a good match for mounting the rotary encoders, but I haven't tried it yet. I did find some nice adjustment knobs for the rotary encoders on eBay. I'd tell you where to get them, but they seem to be some kind of surplus "old stock", so you probably can't get them anyhow. They are made to mount on a "D" shaft, which seems pretty rare these days. Good for me because that's exactly what my rotary encoders have.

For the ceiling mounted PIR sensors, I'm thinking that I might be able to find some pretty inexpensive plastic light fixtures. I can take the guts out of them and then mount the PIR fresnel lens onto it somehow while hiding the actual PIR board inside. I don't want it to look too ugly, but it won't have to be perfect since it's up on the ceiling where people don't tend to look.