The USB Trådfri repeater is probably the smallest option, as suggested above. Like another poster, I'm also not aware of any in-wall switch/outlet repeaters that are compatible--the GE Zigbee switch (they do have one, though most of their line is Z-Wave) is specifically reported as not compatible. An Xbee 3 or S2C is also pretty small but will need to be mounted on some sort of board to power it, but that's also via USB (micro-USB), so your options for hiding it are many, and you can buy or 3D print an enclosure to obscure it more.
If you aren't totally opposed to wall warts but want something less ugly than the Trådfri outlet, the new/2018 SmartThings outlet has worked well for me so far--I think. My routing tables don't always show all my devices, but when I'm able to see Xiaomi devices that don't get paired directly to my hub, they're usually routing through one of these (unfortunately, it's hard to tell after a while since they don't seem to show up on my route info again). Anyway, they're a bit less ugly, though they stick out a bit more than the marketing photos might make you think. They also seem to be fantastic repeaters in general--able to handle more children than most I've seen and apparently quite attractive to most of my devices since they're often favored by devices that I'd think would favor repeaters that are much closer.
Do the ikea outlets act as zigbee repeaters for all zigbee products or just Aqara? If just Aqara seems foolish to corner ones self into that one ecosystem, no?
The ikea outlets act as zigbee repeaters for all zigbee devices.
Some Zigbee repeaters play well with Xiaomi, while others do not. This is do to problems with Xiaomi not being filly compliant with the Zigbee HA 1.2 specification.
As Dan says, they are certified, fully compatible with other devices as well as Xiaomi, which in their current lineup, not many are. TRÅDFRI are decent low cost repeaters, and they are Zigbee 3.0 to boot.
Tradfri are not the strongest so I would guesstimate 30 feet between devices. It’s a radio signal, so materials used in the building, environmental factors, interference, etc. It all plays a role.
It also depends on if you are using the Tradfri repeater, or the outlet. The outlet has much higher power output than the repeater.
I went ahead any just bought 8 of them and spaced them arouny 4k+ sqft house, since it was very easy for me to put them in out of the way locations anyway. Working wonderfully.
Just to add my experience of Zwave v Zigbee to what others have said. I initially had only Zwave plus devices when I first switched from Vera to Hubitat. I had a really bad time with them on my old Vera and while it did improve on Hubitat the range of them was still disappointing and maybe because of that I had a fair bit of unreliability.
If you read up about it Zigbee is meant to have a worse range than Zwave, but when Ikea introduce the Tradfri outlet I bought a few to test and found them at least as good as Zwave if not better.
Fast forward 6 months and I now have more Zigbee devices that Zwave and am gradually trying to phase out Zwave altogether. I find Zigbee faster in response times and easier to join (and can be rejoined easily if need be) and just generally an easier system. There is diagnostic data available on Hubitat too under Zigbee logs so you can see if any of your devices are struggling with signal. With Zwave you have no idea what is going on and why something isn't responding. Another advantage of Zigbee over Zwave for me in the UK is that all devices are the same worldwide, whereas Zwave has different frequencies for different zones.
As others have said you do have to be aware of frequency clashes with Wifi and Phillips Hue if you have it (Also a very good system btw) but once you have them set right there should be no issues.
I wouldn't buy another Zwave device now if there is an equivalent Zigbee one available.
