These are NOT cheap, $52 each, but I love them. Really responsive and they claim up to 5 years on battery. The ceiling one does temp and humidity the other just does motion and temp.
EDIT: Just noticed the desired target is Z-Wave. I'll leave this here just in case. IMHO zigbee is perfect for motion sensors, as it allows them to be small and discreet. Unless you must have Z-wave, strongly recommend these. But to each his/her own!
You can find these on eBay...Iris motion sensors, 3326-L. Fantastic. Small, fast, dependable.
The next gen is a little bigger but adds humidity reporting: IL07 3rd gen. The pic below is not to scale w/the one above. It's just a little bigger. I just bought some of these, $13 each, free shipping.
The above is Zigbee so won't work either. You are less likely to find small Z-Wave motions sensors becsuse until recently, the protcol needed more power than Zigbee, so coin-cell batteries and others without larger capacities were not feasible. The Ring Gen 2 motion sensor uses the current 700-series Z-Wave and, I think, coin cell batteries, or at the very least is a lot smaller than the last generation. However, it requires S2 so will only work if you have a C-7 hub. More may come to the market at some point, but that's the only one I know of now.
I'm not sure what your criteria for "small" are. The Zooz ZSE18, HomeSeer HS-MS100+, Dome DMMS-1, and Fibaro (the one that their marketing photos make look like an eye even though it doesn't) are about golf-ball sized, plus the size of their technically optional mounts. I think they're the smallest I've seen in that arena. Otherwise, as mentioned above, you habe lots of Zigbee options.
if you have a spare pi/linux machine around, i'd highly recommend using [RELEASE] Alpha WyzeSense Integration. it is over zigbee though, but they are small devices
If you donโt have any zigbee devices (ie. repeaters) you can probably still use zigbee motion sensors if your hub is centrally located. When I first set up my second hub, I joined 31 of my zigbee (non-repeating) devices to the hub and left it for a couple days before adding repeaters and other devices. I did this so I would know if I had any end-devices that might be problematic without having to wonder if it was a problem with the repeaters. Even devices on the other end of the house had no problems staying connected.
@Ken_Fraleigh
No, Iโve got a good Zigbee mesh network, but my mailbox is apparently just out of range. Iโm pretty sure a z-wave device would have just enough extra range to work.
@didymus do you have the ability to run a wire to your mailbox? if so, you could try using a WADWAZ-1 and use the external contacts to create a contact sensor on your mailbox door. have it trigger a VS when the mailbox door gets opened, so you'll know when it's been opened