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The Steam Machine is a cute box around the size of a 140mm PC case fan, which means that its motherboard is necessarily tiny. In fact, it's noticeably smaller than a standard ITX size used in DIY small form factor PCs, as you can see in these teardown shots provided by Valve for Digital Foundry.
In conversations with Valve engineers, they mentioned the mainboard is "almost Pico ITX" sized - Pico ITX motherboards measuring 100x72mm (!) - in comparison to a "full-size" Mini ITX motherboard at 170x170mm. The mainboard is a bit larger in terms of raw dimensions, partly due to its mounting points on all four corners protruding further than the circuit board inside, but it's still a feat of engineering.
The image above shows the front of the motherboard on the left, with its AMD CPU (bottom) and GPU (top) clearly visible - albeit under a pile of thermal paste - with four 2GB modules of VRAM surrounding the graphics processor. Flip over to the other side, and you can see the combined CPU/GPU backplate on the top and right, with the single 16GB stick of SODIMM memory (SK Hynix DDR5-5600 if you're interested) in the bottom left. Note that a second RAM slot is available - and the reason it's not populated is interesting.
I'm sure that other publications will do their own teardowns of the device, and it'll be fascinating to see what other nuggets of information are discovered. As we always say with CPU and GPU reviews, we encourage you to read widely to get a balanced idea of performance, as workloads and expectations can vary massively between reviewers.
For now though, why not check out our full Steam Machine review?