NanoPi NEO4 is the Cheapest & Smallest RK3399 Board So Far
https://www.cnx-software.com/2018/10/10/nanopi-neo4-cheapest-smallest-rk3399-board/

NanoPi NEO4 board specifications:

SoC Rockchip RK3399 big.LITTLE hexa-core processor with
　　2x Arm Cortex-A72 @ up to 2.0GHz,
　　4x Cortex-A53 @ up to 1.5GHz,
　　a Mali-T864 GPU with support OpenGL ES1.1/2.0/3.0/3.1, OpenVG1.1, OpenCL, DX11,
　　and AFBC, and a VPU with 4K VP9 and 4K 10-bit H265/H264 decoding
System Memory 1GB DDR3-1866
Storage eMMC module socket, micro SD card slot
Video & Audio Output HDMI 2.0a up to 4K @ 60 Hz with HDCP 1.4/2.2 support
Camera 1x 4-lane MIPI-CSI connector for camera up to 13MP
Connectivity Gigabit Ethernet, 2.4 GHz 802.11 a/b/g/n WiFi,
　　Bluetooth 4.0 with 1x u.FL antenna connector
USB 1x USB 3.0 ports, 1x USB 2.0 type-C port with OTG mode and power input,
　　1x USB 2.0 type A port, 1x USB 2.0 interface via 2.54mm header
Expansion
　　40-pin GPIO1 header with 2x 3V I2C,
　　1x 3V UART/SPI, 1x SPDIF_TX, up to 8x 3V GPIOs,
　　PCIe x2, 1x PWM, PowerKey
　　GPIO2 header with 1x 1.8V 8ch I2S
　　GPIO3 header with debug UART, 1x USB 2.0
Misc Power LED, user LED, 2-pin header for RTC battery
Power Supply 5V/3A via USB-C port; RK808-D PMIC
Dimensions 60 x 45 mm (8-layer PCB)
Temperature Range -20℃ to 70℃