Update (for anyone stumbling on this topic in the search for a small OpenWrt capable device):
I just got an FriendlyARM "NanoPi R5C"
There is also an OpenWrt forum thread.
Seems that in development releases it gets supported by OpenWrt.
It sells for a bit more than 100€ (incl. shipping and tax) for me on AliExpress in the variant with WiFi, case and antennas.
It's like Rockchip RK3568 2.0GHz Quad-core ARM (Cortex-A55), 4GiB RAM, 32 GiB flash, USB 3.2, PCIe slot, and less than 6cm big.
On the low end of computational power, but much smaller, is the VoCore2. Also OpenWrt supported.
