For anyone wanting a very small PC who isn't specifically wanting an ARM processor, I've really had a lot of fun with my Dell/Wyse 5020 Thin Clients. I got a few of them at $15 each from a vendor who had a whole bunch of used ones and was selling them without any accessories (even the power adapter) but there's someone with a bunch of NiB with selling them on eBay for $35 right now: https://www.ebay.com/itm/Dell-Wyse-5020-DX0Q-Thin-Client-1-5GHz-4GB-RAM-16GB-Flash-Keyboard-Mouse-KTHYJ/174279520950?hash=item2893de72b6:g:hdwAAOSwMiFeuVwq
It's not an ARM CPU, but it is a 15W AMD SoC (think like a Cherry Trail Atom but AMD) and is fanless, has 2 SATA connectors, Mini PCIe (that seems not restricted to specific PCI devices, I hooked a Radeon HD 7970 up to mine for kicks through an adapter and it works just fine), USB3, DVI and DisplayPort output, Gigabit Ethernet and 2 slots for RAM, of which one comes populated with 4GB and the SoC actually supports ECC memory as well but haven't tested it myself. It works very well with Linux and I'm new to NetBSD but 9.1 installed with zero issues and works except for the GPU. The GPU works in 9.99-77 though so I'd imagine it will work in stable relatively soon.
They are a good bit bigger than a Pi but still not big and are easily VESA mounted. 190mm X 170mm X 35mm. The boxed ones should also come with the little plastic mount that lets you set it up vertically as well. I think it's a fantastic bargain if you're looking for a quiet power sipping server and not specifically at learning about the ARM platform 🙂