The Raspberry Pi is available in a couple of form factors. There is the standard Model B (85.6 x 56.6mm) with a full complement of I/O connectors. If you are willing to give up some features, you can get the squarish A+ model (65 x 56mm), the Compute Module form factor (67.6 x 30mm), or even the ultra-compact Pi Zero (30 x 65mm).
What do you do if you want to implement a custom Raspberry Pi design? Can something smaller than the Pi Zero be made? The first problem you run into is that the Broadcom SoC is not available to the mass market. That reason alone is why most people would give up on the idea of a custom Raspberry Pi. However, despite not being able to acquire the chips, the team at ArduCam came up with a solution. The result is a 3B+ system-on-module (SoM) measuring only 40 x 25mm or 80% smaller than a traditional Pi!
At first, you might think this board is a Raspberry Pi clone. Since the SoC used on the Pi is not available for others to implement into a custom design, ArduCam is clear they do not plan to sell this as a product. Instead, they explain you’ll need to purchase a Raspberry Pi module and de-solder the SoC. From there, you’ll be able to attach it to the smaller board. (Alternatively, they offer services to do that part for you, provided, you provide the Pi.)
Make sure you check out the demo video. It shows how they have attached the SoM to a carrier board. Using their cameras, they demo a dual camera setup after the Pi successfully boots.